diff --git a/tests/test_components/test_medium.py b/tests/test_components/test_medium.py
index d6954241c3..a2507b5007 100644
--- a/tests/test_components/test_medium.py
+++ b/tests/test_components/test_medium.py
@@ -433,6 +433,9 @@ def test_medium2d(log_capture):  # noqa: F811
     plt.close()
     assert_log_level(log_capture, "WARNING")
 
+    with pytest.raises(pydantic.ValidationError):
+        _ = td.Medium2D(ss=td.PECMedium(), tt=td.Medium())
+
 
 def test_rotation():
     # check that transpose is inverse
diff --git a/tidy3d/components/medium.py b/tidy3d/components/medium.py
index b9623ed577..0326410b03 100644
--- a/tidy3d/components/medium.py
+++ b/tidy3d/components/medium.py
@@ -5473,6 +5473,17 @@ def _validate_modulation_spec(cls, val):
             )
         return val
 
+    @skip_if_fields_missing(["ss"])
+    @pd.validator("tt", always=True)
+    def _validate_inplane_pec(cls, val, values):
+        """ss/tt components must be both PEC or non-PEC."""
+        if isinstance(val, PECMedium) != isinstance(values["ss"], PECMedium):
+            raise ValidationError(
+                "Materials describing ss- and tt-components must be "
+                "either both 'PECMedium', or non-'PECMedium'."
+            )
+        return val
+
     @classmethod
     def _weighted_avg(
         cls, meds: List[IsotropicUniformMediumType], weights: List[float]