diff --git a/src/power_grid_model/validation/rules.py b/src/power_grid_model/validation/rules.py
index 9e8553787f..b4edd15654 100644
--- a/src/power_grid_model/validation/rules.py
+++ b/src/power_grid_model/validation/rules.py
@@ -400,7 +400,7 @@ def all_valid_enum_values(
         was not a valid value in the supplied enum type.
     """
     valid = [nan_type(component, field)] + list(enum)
-    invalid = np.isin(data[component][field], valid, invert=True)
+    invalid = np.isin(data[component][field], np.array(valid, dtype=np.int8), invert=True)
     if invalid.any():
         ids = data[component]["id"][invalid].flatten().tolist()
         return [InvalidEnumValueError(component, field, ids, enum)]
diff --git a/tests/unit/validation/test_rules.py b/tests/unit/validation/test_rules.py
index c75ddeafe7..e9c6f612e2 100644
--- a/tests/unit/validation/test_rules.py
+++ b/tests/unit/validation/test_rules.py
@@ -266,6 +266,11 @@ class MyEnum(IntEnum):
         alpha = 2
         bravo = 5
 
+    # TODO replace this hack with some patch to power_grid_meta_data or nan_type
+    from power_grid_model import power_grid_meta_data
+
+    power_grid_meta_data["input"]["test"] = {"nans": {"value": -128}}
+
     valid = {"test": np.array([(1, 2), (2, 5)], dtype=[("id", "i4"), ("value", "i4")])}
     errors = all_valid_enum_values(valid, "test", "value", MyEnum)
     assert not errors
@@ -274,6 +279,18 @@ class MyEnum(IntEnum):
     errors = all_valid_enum_values(invalid, "test", "value", MyEnum)
     assert len(errors) == 1
     assert InvalidEnumValueError("test", "value", [2], MyEnum) in errors
+    # TODO replace this hack with some patch to power_grid_meta_data or nan_type
+    del power_grid_meta_data["input"]["test"]
+
+    # try with a real enum LoadGenType
+    # this is a bug in numpy
+    from power_grid_model import LoadGenType, initialize_array
+
+    valid = {"sym_load": initialize_array("input", "sym_load", 20)}
+    valid["sym_load"]["id"] = np.arange(20)
+    valid["sym_load"]["type"] = 0
+    errors = all_valid_enum_values(valid, "sym_load", "type", LoadGenType)
+    assert not errors
 
 
 def test_all_valid_ids():