diff --git a/custom_components/luxtronik2/config_flow.py b/custom_components/luxtronik2/config_flow.py
index 8e8a2382..5813f29a 100644
--- a/custom_components/luxtronik2/config_flow.py
+++ b/custom_components/luxtronik2/config_flow.py
@@ -25,7 +25,7 @@
     DEFAULT_MAX_DATA_LENGTH,
     DEFAULT_PORT,
     DEFAULT_TIMEOUT,
-    DEFAULT_UPDATE_INTERVAL,
+    DEFAULT_UPDATE_INTERVAL_OPTION,
     DOMAIN,
     LOGGER,
 )
@@ -512,7 +512,7 @@ async def async_step_user(
                     new_options[CONF_HA_SENSOR_INDOOR_TEMPERATURE] = None
 
                 update_interval = user_input.get(
-                    CONF_UPDATE_INTERVAL, DEFAULT_UPDATE_INTERVAL
+                    CONF_UPDATE_INTERVAL, DEFAULT_UPDATE_INTERVAL_OPTION
                 )
                 new_options[CONF_UPDATE_INTERVAL] = update_interval
 
@@ -520,7 +520,7 @@ async def async_step_user(
 
             current_indoor_temp = self._get_value(CONF_HA_SENSOR_INDOOR_TEMPERATURE)
             current_interval = self._get_value(
-                CONF_UPDATE_INTERVAL, DEFAULT_UPDATE_INTERVAL
+                CONF_UPDATE_INTERVAL, DEFAULT_UPDATE_INTERVAL_OPTION
             )
 
             return self.async_show_form(
diff --git a/custom_components/luxtronik2/const.py b/custom_components/luxtronik2/const.py
index 8167037c..d443c45a 100644
--- a/custom_components/luxtronik2/const.py
+++ b/custom_components/luxtronik2/const.py
@@ -42,6 +42,15 @@
     "1 minute (default)": timedelta(seconds=60),
     "5 minutes": timedelta(minutes=5),
 }
+# The UPDATE_INTERVAL_OPTIONS key matching DEFAULT_UPDATE_INTERVAL. The options
+# flow's CONF_UPDATE_INTERVAL field is a SelectSelector whose default/suggested
+# value must be one of these string keys - DEFAULT_UPDATE_INTERVAL itself is a
+# timedelta and isn't JSON-serializable when used there.
+DEFAULT_UPDATE_INTERVAL_OPTION: Final = next(
+    key
+    for key, value in UPDATE_INTERVAL_OPTIONS.items()
+    if value == DEFAULT_UPDATE_INTERVAL
+)
 
 UPDATE_INTERVAL_FAST: Final = timedelta(seconds=10)
 UPDATE_INTERVAL_NORMAL: Final = timedelta(minutes=1)
diff --git a/custom_components/luxtronik2/schema_helper.py b/custom_components/luxtronik2/schema_helper.py
index e5409701..6d1be5c8 100644
--- a/custom_components/luxtronik2/schema_helper.py
+++ b/custom_components/luxtronik2/schema_helper.py
@@ -10,7 +10,7 @@
     DEFAULT_MAX_DATA_LENGTH,
     DEFAULT_PORT,
     DEFAULT_TIMEOUT,
-    DEFAULT_UPDATE_INTERVAL,
+    DEFAULT_UPDATE_INTERVAL_OPTION,
     UPDATE_INTERVAL_OPTIONS,
 )
 
@@ -53,9 +53,10 @@ def build_options_schema(
             ),
             vol.Optional(
                 CONF_UPDATE_INTERVAL,
-                default=current_interval or DEFAULT_UPDATE_INTERVAL,
+                default=current_interval or DEFAULT_UPDATE_INTERVAL_OPTION,
                 description={
-                    "suggested_value": current_interval or DEFAULT_UPDATE_INTERVAL
+                    "suggested_value": current_interval
+                    or DEFAULT_UPDATE_INTERVAL_OPTION
                 },
             ): selector.SelectSelector(
                 selector.SelectSelectorConfig(
diff --git a/tests/test_const.py b/tests/test_const.py
index 93887386..17216e8d 100644
--- a/tests/test_const.py
+++ b/tests/test_const.py
@@ -14,6 +14,7 @@
     DEFAULT_PORT,
     DEFAULT_TIMEOUT,
     DEFAULT_UPDATE_INTERVAL,
+    DEFAULT_UPDATE_INTERVAL_OPTION,
     DOMAIN,
     PLATFORMS,
     UPDATE_INTERVAL_OPTIONS,
@@ -198,6 +199,19 @@ def test_update_interval_options_keys_and_timedeltas(self):
         assert UPDATE_INTERVAL_OPTIONS["1 minute (default)"].total_seconds() == 60
         assert UPDATE_INTERVAL_OPTIONS["5 minutes"].total_seconds() == 300
 
+    def test_default_update_interval_option_is_a_valid_key(self):
+        """DEFAULT_UPDATE_INTERVAL_OPTION must be a string key of
+        UPDATE_INTERVAL_OPTIONS, not the DEFAULT_UPDATE_INTERVAL timedelta
+        itself - the options flow's SelectSelector default must match one of
+        its own option values or the schema fails to serialize (see #656).
+        """
+        assert isinstance(DEFAULT_UPDATE_INTERVAL_OPTION, str)
+        assert DEFAULT_UPDATE_INTERVAL_OPTION in UPDATE_INTERVAL_OPTIONS
+        assert (
+            UPDATE_INTERVAL_OPTIONS[DEFAULT_UPDATE_INTERVAL_OPTION]
+            == DEFAULT_UPDATE_INTERVAL
+        )
+
     def test_conf_update_interval_constant(self):
         assert CONF_UPDATE_INTERVAL == "update_interval"
 
diff --git a/tests/test_schema_helper.py b/tests/test_schema_helper.py
index 519348f3..0e3fadd2 100644
--- a/tests/test_schema_helper.py
+++ b/tests/test_schema_helper.py
@@ -2,11 +2,14 @@
 
 from __future__ import annotations
 
+import json
 from typing import Any, cast
 
 from homeassistant.const import CONF_HOST, CONF_PORT
+import homeassistant.helpers.config_validation as cv
 import pytest
 import voluptuous as vol
+import voluptuous_serialize
 
 from custom_components.luxtronik2.const import (
     DEFAULT_PORT,
@@ -92,3 +95,18 @@ def test_with_update_interval(self):
             current_interval="1 minute",
         )
         assert isinstance(schema, vol.Schema)
+
+    def test_default_schema_is_json_serializable(self):
+        """Regression test for issue #656.
+
+        A config entry that has never saved options before (current_interval=None,
+        the common case) must not fall back to the raw DEFAULT_UPDATE_INTERVAL
+        timedelta - the frontend serializes this schema to JSON to render the
+        "Configure" form, and a timedelta default/suggested_value crashes that
+        with a 500 (TypeError: Object of type timedelta is not JSON serializable).
+        """
+        schema = build_options_schema()
+        converted = voluptuous_serialize.convert(
+            schema, custom_serializer=cv.custom_serializer
+        )
+        json.dumps(converted)  # must not raise