diff --git a/src/openjd/model/v2023_09/_model.py b/src/openjd/model/v2023_09/_model.py
index b0a8837..890b7ed 100644
--- a/src/openjd/model/v2023_09/_model.py
+++ b/src/openjd/model/v2023_09/_model.py
@@ -1055,6 +1055,8 @@ class Environment(OpenJDModel_v2023_09):
     @model_validator(mode="before")
     @classmethod
     def _validate_has_script_or_variables(cls, values: dict[str, Any]) -> dict[str, Any]:
+        if not isinstance(values, dict):
+            raise ValueError("Environment must be a mapping.")
         if values.get("script") is None and values.get("variables") is None:
             raise ValueError("Environment must have either a script or variables.")
         return values
diff --git a/test/openjd/model/v2023_09/test_environment_template.py b/test/openjd/model/v2023_09/test_environment_template.py
index edef1b0..864e134 100644
--- a/test/openjd/model/v2023_09/test_environment_template.py
+++ b/test/openjd/model/v2023_09/test_environment_template.py
@@ -102,6 +102,14 @@ def test_parse_success(self, data: dict[str, Any]) -> None:
                 1,
                 id="missing spec ver",
             ),
+            pytest.param(
+                {
+                    "specificationVersion": "environment-2023-09",
+                    "environment": None,
+                },
+                1,
+                id="environment is None",
+            ),
             #
             pytest.param(
                 {