diff --git a/cognite/client/data_classes/simulators/routine_revisions.py b/cognite/client/data_classes/simulators/routine_revisions.py
index 20e2c6c0e8..e7d62116e4 100644
--- a/cognite/client/data_classes/simulators/routine_revisions.py
+++ b/cognite/client/data_classes/simulators/routine_revisions.py
@@ -83,8 +83,13 @@ def _load(
 
         is_constant = resource.get("value")
         is_timeseries = resource.get("sourceExternalId")
-        type_ = "constant" if is_constant else "timeseries" if is_timeseries else None
-        if type_ is None:
+        if is_constant is not None and is_timeseries is not None:
+            raise ValueError("Invalid routine input, cannot contain both 'value' and 'sourceExternalId'")
+        elif is_constant is not None:
+            type_ = "constant"
+        elif is_timeseries is not None:
+            type_ = "timeseries"
+        else:
             return UnknownCogniteObject(resource)  # type: ignore[return-value]
         input_class = _INPUT_CLASS_BY_TYPE.get(type_)
         if input_class is None:
diff --git a/tests/tests_unit/test_data_classes/test_simulators.py b/tests/tests_unit/test_data_classes/test_simulators.py
index 33c211680a..9b98789d09 100644
--- a/tests/tests_unit/test_data_classes/test_simulators.py
+++ b/tests/tests_unit/test_data_classes/test_simulators.py
@@ -11,6 +11,7 @@
 from cognite.client.data_classes.simulators.routine_revisions import (
     SimulationValueUnitInput,
     SimulatorRoutineConfiguration,
+    SimulatorRoutineInput,
     SimulatorRoutineInputConstant,
     SimulatorRoutineInputTimeseries,
     SimulatorRoutineOutput,
@@ -489,3 +490,24 @@ def test_error_handling(self) -> None:
             SimulationRunWrite(
                 routine_revision_external_id="routine_revision_external_id_1",
             )
+
+
+class TestSimulatorRoutineInput:
+    def test_load_zero_value(self) -> None:
+        routine_input = SimulatorRoutineInput._load(
+            {"name": "input", "referenceId": "input", "value": 0, "valueType": "DOUBLE"}
+        )
+        assert isinstance(routine_input, SimulatorRoutineInputConstant)
+        assert routine_input.value == 0
+
+    def test_load_empty_string_source_external_id(self) -> None:
+        routine_input = SimulatorRoutineInput._load({"name": "input", "referenceId": "input", "sourceExternalId": ""})
+        assert isinstance(routine_input, SimulatorRoutineInputTimeseries)
+        assert routine_input.source_external_id == ""
+
+    def test_load_both_value_and_time_series(self) -> None:
+        with pytest.raises(
+            ValueError,
+            match="Invalid routine input, cannot contain both 'value' and 'sourceExternalId'",
+        ):
+            SimulatorRoutineInput._load({"name": "input", "referenceId": "input", "sourceExternalId": "", "value": 0})