diff --git a/docs/docs/settings/error_codes.md b/docs/docs/settings/error_codes.md
index e444125fdc26..bfd72f93990b 100644
--- a/docs/docs/settings/error_codes.md
+++ b/docs/docs/settings/error_codes.md
@@ -110,6 +110,14 @@ While using [invoke](../start/invoke.md), this can be overridden with the `--res
 
 An error occurred while rendering a component in the frontend. Typically this is caused by a browser caching issue, and can be resolved by clearing the browser cache and refreshing the page. If the issue persists, check the browser console for more information about the error.
 
+#### INVE-E18
+
+**Top Level Domain Required**
+
+The InvenTree server failed to start, because the configured server URL does not include a top-level domain (TLD). A valid TLD is required for proper operation.
+
+## Warning Codes
+
 ### INVE-W (InvenTree Warning)
 Warnings - These are non-critical errors which should be addressed when possible.
 
diff --git a/src/backend/InvenTree/InvenTree/settings.py b/src/backend/InvenTree/InvenTree/settings.py
index cb691c2685a9..fe85764553cd 100644
--- a/src/backend/InvenTree/InvenTree/settings.py
+++ b/src/backend/InvenTree/InvenTree/settings.py
@@ -28,6 +28,7 @@
 from InvenTree.config import get_boolean_setting, get_oidc_private_key, get_setting
 from InvenTree.ready import isInMainThread, isRunningBackup
 from InvenTree.sentry import default_sentry_dsn, init_sentry
+from InvenTree.validators import invalid_site_url_hint
 from InvenTree.version import checkMinPythonVersion, inventreeCommitHash
 from users.oauth2_scopes import oauth2_scopes
 
@@ -771,7 +772,7 @@
         validator = URLValidator()
         validator(SITE_URL)
     except Exception:
-        msg = f"Invalid SITE_URL value: '{SITE_URL}'. InvenTree server cannot start."
+        msg = f"Invalid SITE_URL value: '{SITE_URL}'. InvenTree server cannot start.{invalid_site_url_hint(SITE_URL)}"
         logger.error(msg)
         print(msg)
         sys.exit(-1)
diff --git a/src/backend/InvenTree/InvenTree/validators.py b/src/backend/InvenTree/InvenTree/validators.py
index ae4145a140ae..13ab702e07a7 100644
--- a/src/backend/InvenTree/InvenTree/validators.py
+++ b/src/backend/InvenTree/InvenTree/validators.py
@@ -1,6 +1,8 @@
 """Custom field validators for InvenTree."""
 
+import ipaddress
 import tokenize
+from urllib.parse import urlsplit
 
 from django.conf import settings
 from django.core import validators
@@ -88,6 +90,39 @@ def __call__(self, value):
         super().__call__(value)
 
 
+def invalid_site_url_hint(site_url: str) -> str:
+    """Return an extra hint for *why* a SITE_URL value failed validation.
+
+    Django's URLValidator rejects any hostname that isn't 'localhost', an IP
+    address, or a fully qualified (dotted) name - so a bare LAN hostname like
+    'warehouse' fails with no indication of what's actually wrong. Returns an
+    empty string when nothing more specific than "invalid URL" applies.
+    """
+    hostname = urlsplit(site_url).hostname
+
+    if hostname is None and '//' not in site_url:
+        # No scheme was given at all, e.g. SITE_URL=warehouse
+        hostname = urlsplit(f'//{site_url}').hostname
+
+    if not hostname or hostname == 'localhost':
+        return ''
+
+    try:
+        ipaddress.ip_address(hostname)
+        return ''
+    except ValueError:
+        pass
+
+    if '.' not in hostname:
+        return (
+            f"INVE-E17: Top Level Domain Required.\ni'{hostname}' has no top-level domain. InvenTree requires a fully "
+            "qualified hostname (e.g. 'warehouse.local'), an IP address, or "
+            "'localhost' - a bare hostname is rejected by Django's URL validator."
+        )
+
+    return ''
+
+
 def validate_purchase_order_reference(value):
     """Validate the 'reference' field of a PurchaseOrder."""
     from order.models import PurchaseOrder