diff --git a/client/src/components/BatchPanel.tsx b/client/src/components/BatchPanel.tsx
index 5846254..fe3cb6c 100644
--- a/client/src/components/BatchPanel.tsx
+++ b/client/src/components/BatchPanel.tsx
@@ -180,7 +180,7 @@ export function BatchPanel() {
                           </td>
                         ))}
                         <td className="border border-gray-200 px-1 py-0.5">
-                          <div className="flex items-center justify-center gap-2">
+                          <div className="flex items-center justify-center gap-4">
                             <button
                               className={
                                 isPreviewed
diff --git a/client/src/components/NumberField.test.tsx b/client/src/components/NumberField.test.tsx
new file mode 100644
index 0000000..1b347a0
--- /dev/null
+++ b/client/src/components/NumberField.test.tsx
@@ -0,0 +1,128 @@
+import { describe, it, expect, afterEach, vi } from "vitest";
+import { render, screen, cleanup } from "@testing-library/react";
+import userEvent from "@testing-library/user-event";
+import { useState } from "react";
+
+import { NumberField } from "./NumberField";
+
+afterEach(() => {
+  cleanup();
+});
+
+/** Controlled wrapper so the displayed value reflects committed changes,
+ *  mirroring how TextWidgetEditor drives NumberField through the store. */
+function Harness({
+  initial = 90,
+  min = 10,
+  max = 150,
+  onCommit,
+}: {
+  initial?: number;
+  min?: number;
+  max?: number;
+  onCommit?: (n: number) => void;
+}) {
+  const [value, setValue] = useState(initial);
+  return (
+    <NumberField
+      label="Scale %"
+      value={value}
+      min={min}
+      max={max}
+      onChange={(n) => {
+        setValue(n);
+        onCommit?.(n);
+      }}
+    />
+  );
+}
+
+describe("NumberField", () => {
+  it("clearing the field shows empty, not 0, and commits nothing", async () => {
+    const user = userEvent.setup();
+    const onCommit = vi.fn();
+    render(<Harness onCommit={onCommit} />);
+    const input = screen.getByRole("spinbutton") as HTMLInputElement;
+
+    await user.clear(input);
+
+    expect(input.value).toBe("");
+    expect(onCommit).not.toHaveBeenCalledWith(0);
+  });
+
+  it("lets you clear and type a fresh value without a leading zero", async () => {
+    const user = userEvent.setup();
+    const onCommit = vi.fn();
+    render(<Harness onCommit={onCommit} />);
+    const input = screen.getByRole("spinbutton") as HTMLInputElement;
+
+    await user.clear(input);
+    await user.type(input, "60");
+
+    expect(input.value).toBe("60");
+    expect(onCommit).toHaveBeenLastCalledWith(60);
+  });
+
+  it("clamps a committed value below the minimum", async () => {
+    const user = userEvent.setup();
+    const onCommit = vi.fn();
+    render(<Harness min={10} onCommit={onCommit} />);
+    const input = screen.getByRole("spinbutton") as HTMLInputElement;
+
+    await user.clear(input);
+    await user.type(input, "6");
+
+    expect(onCommit).toHaveBeenLastCalledWith(10);
+  });
+
+  it("clamps a committed value above the maximum", async () => {
+    const user = userEvent.setup();
+    const onCommit = vi.fn();
+    render(<Harness max={150} onCommit={onCommit} />);
+    const input = screen.getByRole("spinbutton") as HTMLInputElement;
+
+    await user.clear(input);
+    await user.type(input, "999");
+
+    expect(onCommit).toHaveBeenLastCalledWith(150);
+  });
+
+  it("settles the field to the committed (clamped) value on blur", async () => {
+    const user = userEvent.setup();
+    render(<Harness min={10} />);
+    const input = screen.getByRole("spinbutton") as HTMLInputElement;
+
+    await user.clear(input);
+    await user.type(input, "6");
+    expect(input.value).toBe("6"); // not fought while typing
+    await user.tab();
+
+    expect(input.value).toBe("10");
+  });
+
+  it("reflects an external value change when not being edited", async () => {
+    function Rerenderer() {
+      const [value, setValue] = useState(90);
+      return (
+        <>
+          <button onClick={() => setValue(120)}>set</button>
+          <NumberField
+            label="Scale %"
+            value={value}
+            min={10}
+            max={150}
+            onChange={() => {}}
+          />
+        </>
+      );
+    }
+    const user = userEvent.setup();
+    render(<Rerenderer />);
+    const input = screen.getByRole("spinbutton") as HTMLInputElement;
+    expect(input.value).toBe("90");
+
+    await user.click(screen.getByText("set"));
+
+    expect(input.value).toBe("120");
+  });
+});
diff --git a/client/src/components/NumberField.tsx b/client/src/components/NumberField.tsx
new file mode 100644
index 0000000..8d5cf74
--- /dev/null
+++ b/client/src/components/NumberField.tsx
@@ -0,0 +1,66 @@
+import { useEffect, useState } from "react";
+
+type NumberFieldProps = {
+  label: string;
+  value: number;
+  min: number;
+  max: number;
+  className?: string;
+  onChange: (value: number) => void;
+};
+
+/**
+ * A numeric input that's pleasant to hand-edit.
+ *
+ * A plain controlled `<input type="number" value={number}>` fights the user:
+ * clearing it yields `Number("") === 0` (so the field jumps to 0, and the
+ * server then has to defend against a 0-size font), and the leftover leading
+ * digit makes the next keystroke read as e.g. "06". Here we keep the visible
+ * text as a free-form draft so the field can be emptied and retyped, commit a
+ * clamped number to the parent only when the draft parses, and resync the draft
+ * to the committed value on blur — never while typing, so clamping can't snap a
+ * half-entered number out from under the cursor.
+ */
+export function NumberField({
+  label,
+  value,
+  min,
+  max,
+  className,
+  onChange,
+}: NumberFieldProps) {
+  const [draft, setDraft] = useState(String(value));
+  const [focused, setFocused] = useState(false);
+
+  // Mirror an external value change (e.g. loading a label file) into the field,
+  // but stay out of the way while the user is actively editing it.
+  useEffect(() => {
+    if (!focused) setDraft(String(value));
+  }, [value, focused]);
+
+  const clamp = (n: number) => Math.min(max, Math.max(min, n));
+
+  return (
+    <label className="flex items-center gap-1 text-xs">
+      {label}
+      <input
+        type="number"
+        className={className}
+        min={min}
+        max={max}
+        value={draft}
+        onFocus={() => setFocused(true)}
+        onBlur={() => setFocused(false)}
+        onChange={(e) => {
+          const raw = e.target.value;
+          setDraft(raw);
+          // Empty or non-numeric mid-edit: hold the last committed value so the
+          // preview never renders an invalid size. The field settles on blur.
+          if (raw.trim() === "") return;
+          const parsed = Number(raw);
+          if (!Number.isNaN(parsed)) onChange(clamp(parsed));
+        }}
+      />
+    </label>
+  );
+}
diff --git a/client/src/components/TextWidgetEditor.tsx b/client/src/components/TextWidgetEditor.tsx
index ba51a8f..7f7df8d 100644
--- a/client/src/components/TextWidgetEditor.tsx
+++ b/client/src/components/TextWidgetEditor.tsx
@@ -1,5 +1,6 @@
 import { useLabelStore } from "../state/useLabelStore";
 import type { TextWidget, FontStyle, Alignment } from "../types/label";
+import { NumberField } from "./NumberField";
 
 export function TextWidgetEditor({ widget }: { widget: TextWidget }) {
   const update = useLabelStore((s) => s.updateWidget);
@@ -30,33 +31,23 @@ export function TextWidgetEditor({ widget }: { widget: TextWidget }) {
           </select>
         </label>
 
-        <label className="flex items-center gap-1 text-xs">
-          Scale %
-          <input
-            type="number"
-            className="input text-xs w-16"
-            min={10}
-            max={150}
-            value={widget.fontScale}
-            onChange={(e) =>
-              update(widget.id, { fontScale: Number(e.target.value) })
-            }
-          />
-        </label>
+        <NumberField
+          label="Scale %"
+          className="input text-xs w-16"
+          min={10}
+          max={150}
+          value={widget.fontScale}
+          onChange={(fontScale) => update(widget.id, { fontScale })}
+        />
 
-        <label className="flex items-center gap-1 text-xs">
-          Frame
-          <input
-            type="number"
-            className="input text-xs w-14"
-            min={0}
-            max={20}
-            value={widget.frameWidthPx}
-            onChange={(e) =>
-              update(widget.id, { frameWidthPx: Number(e.target.value) })
-            }
-          />
-        </label>
+        <NumberField
+          label="Frame"
+          className="input text-xs w-14"
+          min={0}
+          max={20}
+          value={widget.frameWidthPx}
+          onChange={(frameWidthPx) => update(widget.id, { frameWidthPx })}
+        />
 
         <label className="flex items-center gap-1 text-xs">
           Align
diff --git a/server/label_builder.py b/server/label_builder.py
index 403eadc..c67b4cd 100644
--- a/server/label_builder.py
+++ b/server/label_builder.py
@@ -1,3 +1,4 @@
+import math
 import os
 from io import BytesIO
 
@@ -31,6 +32,39 @@ def mm_to_payload_px(mm: float, margin: float) -> float:
     return max(0, (mm * PIXELS_PER_MM) - margin * 2)
 
 
+# Text scale as a percentage; matches the client default (DEFAULT_FONT_SCALE in
+# client/src/lib/constants.ts). Used as the fallback when a widget carries an
+# unusable fontScale.
+DEFAULT_FONT_SCALE = 90
+
+
+def _font_size_ratio(widget: dict) -> float:
+    """A text widget's fontScale (%) as a positive ratio for TextRenderEngine.
+
+    labelle sizes the font as round(line_height * ratio) and PIL rejects a
+    size of 0, so a non-positive or non-numeric fontScale would 500 the whole
+    render. The UI produces exactly that the instant the Scale field is cleared
+    (Number("") === 0), and a malformed label file or direct API call can carry
+    it too — so fall back to the default rather than crash. See #49.
+
+    NaN and Infinity get the same treatment: Flask's JSON parser accepts the
+    `NaN`/`Infinity` literals, and both slip past a plain `<= 0` check (then
+    blow up in PIL's int conversion), so guard with `math.isfinite`.
+
+    Note: a tiny *positive* fontScale can still round to 0 px on a small tape
+    (or with many lines) and raise. That's out of reach from the UI (min 10)
+    and tape-dependent, so it isn't normalized here; see #49 for the residual.
+    """
+    raw = widget.get("fontScale", DEFAULT_FONT_SCALE)
+    try:
+        scale = float(raw)
+    except (TypeError, ValueError):
+        scale = DEFAULT_FONT_SCALE
+    if not math.isfinite(scale) or scale <= 0:
+        scale = DEFAULT_FONT_SCALE
+    return scale / 100.0
+
+
 # Cut-mark pattern. CUT_MARK_ON pixels on, CUT_MARK_OFF off, repeated for the
 # tape's full height. A column of dotted pixels painted into the trailing
 # margin of each batch label (except the last) so the user can tear/cut
@@ -101,7 +135,7 @@ def _build_render_engines(
                     text_lines=text.split("\n"),
                     font_file_name=font_path,
                     frame_width_px=widget.get("frameWidthPx", 0),
-                    font_size_ratio=widget.get("fontScale", 90) / 100.0,
+                    font_size_ratio=_font_size_ratio(widget),
                     align=Direction(widget.get("align", "left")),
                 )
             )
diff --git a/server/tests/test_label_builder.py b/server/tests/test_label_builder.py
index 17fe7c5..755e051 100644
--- a/server/tests/test_label_builder.py
+++ b/server/tests/test_label_builder.py
@@ -5,6 +5,7 @@
 from PIL import Image
 
 from label_builder import (
+    DEFAULT_FONT_SCALE,
     _build_render_engines,
     mm_to_payload_px,
     preview_label,
@@ -117,6 +118,59 @@ def test_invalid_barcode_type_falls_back_to_default(self):
         assert isinstance(engines[0], BarcodeRenderEngine)
 
 
+class TestFontScaleGuard:
+    """A non-positive or non-numeric fontScale must not crash rendering.
+
+    labelle computes the pixel font size as round(line_height * fontScale/100)
+    and PIL's ImageFont rejects a size of 0, so a fontScale of 0 — which the UI
+    produces the moment the Scale field is cleared (Number("") === 0), and which
+    a malformed label file or direct API call can also carry — would otherwise
+    500 the whole preview/print. We fall back to the default instead. See #49."""
+
+    def test_zero_font_scale_falls_back_to_default(self):
+        widgets = [{"type": "text", "text": "Hello", "id": "1", "fontScale": 0}]
+        engines = _build_render_engines(widgets)
+        assert engines[0].font_size_ratio == DEFAULT_FONT_SCALE / 100.0
+
+    def test_negative_font_scale_falls_back_to_default(self):
+        widgets = [{"type": "text", "text": "Hello", "id": "1", "fontScale": -20}]
+        engines = _build_render_engines(widgets)
+        assert engines[0].font_size_ratio == DEFAULT_FONT_SCALE / 100.0
+
+    def test_non_numeric_font_scale_falls_back_to_default(self):
+        widgets = [{"type": "text", "text": "Hello", "id": "1", "fontScale": None}]
+        engines = _build_render_engines(widgets)
+        assert engines[0].font_size_ratio == DEFAULT_FONT_SCALE / 100.0
+
+    def test_nan_and_infinity_fall_back_to_default(self):
+        # Flask's JSON parser accepts the NaN/Infinity literals, and both slip
+        # past a plain `<= 0` check before exploding in PIL's int conversion.
+        for bad in (float("nan"), float("inf"), float("-inf")):
+            widgets = [{"type": "text", "text": "Hi", "id": "1", "fontScale": bad}]
+            engines = _build_render_engines(widgets)
+            assert engines[0].font_size_ratio == DEFAULT_FONT_SCALE / 100.0
+
+    def test_preview_with_nan_font_scale_does_not_raise(self):
+        widgets = [{"type": "text", "text": "Hi", "id": "1", "fontScale": float("nan")}]
+        result = preview_label(widgets, {"tapeSizeMm": 6})
+        assert result[:8] == b"\x89PNG\r\n\x1a\n"
+
+    def test_valid_font_scale_is_preserved(self):
+        widgets = [{"type": "text", "text": "Hello", "id": "1", "fontScale": 50}]
+        engines = _build_render_engines(widgets)
+        assert engines[0].font_size_ratio == 0.5
+
+    def test_missing_font_scale_uses_default(self):
+        widgets = [{"type": "text", "text": "Hello", "id": "1"}]
+        engines = _build_render_engines(widgets)
+        assert engines[0].font_size_ratio == DEFAULT_FONT_SCALE / 100.0
+
+    def test_preview_with_zero_font_scale_does_not_raise(self):
+        widgets = [{"type": "text", "text": "Hello", "id": "1", "fontScale": 0}]
+        result = preview_label(widgets, {"tapeSizeMm": 12})
+        assert result[:8] == b"\x89PNG\r\n\x1a\n"
+
+
 class TestPreviewLabel:
     def test_returns_valid_png_bytes(self):
         widgets = [{"type": "text", "text": "Test", "id": "1"}]