diff --git a/RELEASE_NOTES.md b/RELEASE_NOTES.md
index 61ee6f2ad..412c6f25e 100644
--- a/RELEASE_NOTES.md
+++ b/RELEASE_NOTES.md
@@ -6,6 +6,8 @@
 
 ## Upgrading
 
+- Custom validators passed through a configuration dataclass field's `metadata` (as used by `load_config()` and `ConfigManager.new_receiver()`) must now raise a `marshmallow.ValidationError` to reject a value. `marshmallow` 4 removed support for validators that signal failure by returning `False`, so such validators are now ignored and the invalid value is accepted silently. This affects you as soon as `marshmallow` 4 is installed, which `frequenz-quantities` 1.0.2 and later require.
+
 <!-- Here goes notes on how to upgrade from previous versions, including deprecations and what they should be replaced with -->
 
 ## New Features
diff --git a/tests/config/test_manager.py b/tests/config/test_manager.py
index 7805eef5a..3a4341015 100644
--- a/tests/config/test_manager.py
+++ b/tests/config/test_manager.py
@@ -20,11 +20,24 @@
 from frequenz.sdk.config._manager import _get_key
 
 
+def _validate_name(value: str) -> None:
+    """Validate that a name starts with `test`.
+
+    Args:
+        value: The name to validate.
+
+    Raises:
+        marshmallow.ValidationError: If the name doesn't start with `test`.
+    """
+    if not value.startswith("test"):
+        raise marshmallow.ValidationError("Invalid value.")
+
+
 @dataclass
 class SimpleConfig:
     """A simple configuration class for testing."""
 
-    name: str = dataclasses.field(metadata={"validate": lambda s: s.startswith("test")})
+    name: str = dataclasses.field(metadata={"validate": _validate_name})
     value: int
 
 
diff --git a/tests/config/test_util.py b/tests/config/test_util.py
index 13283c165..c5beaa962 100644
--- a/tests/config/test_util.py
+++ b/tests/config/test_util.py
@@ -14,11 +14,24 @@
 from frequenz.sdk.config._util import load_config
 
 
+def _validate_name(value: str) -> None:
+    """Validate that a name starts with `test`.
+
+    Args:
+        value: The name to validate.
+
+    Raises:
+        marshmallow.ValidationError: If the name doesn't start with `test`.
+    """
+    if not value.startswith("test"):
+        raise marshmallow.ValidationError("Invalid value.")
+
+
 @dataclasses.dataclass
 class SimpleConfig:
     """A simple configuration class for testing."""
 
-    name: str = dataclasses.field(metadata={"validate": lambda s: s.startswith("test")})
+    name: str = dataclasses.field(metadata={"validate": _validate_name})
     value: int
 
 
@@ -26,7 +39,7 @@ class SimpleConfig:
 class MmSimpleConfig:
     """A simple marshmallow_dataclass configuration class for testing."""
 
-    name: str = dataclasses.field(metadata={"validate": lambda s: s.startswith("test")})
+    name: str = dataclasses.field(metadata={"validate": _validate_name})
     value: int