diff --git a/src/tabmat/formula.py b/src/tabmat/formula.py
index dc8ce6b6..ed7bd2dc 100644
--- a/src/tabmat/formula.py
+++ b/src/tabmat/formula.py
@@ -10,7 +10,7 @@
 import pandas
 from formulaic import ModelMatrix, ModelSpec
 from formulaic.errors import FactorEncodingError
-from formulaic.materializers import NarwhalsMaterializer
+from formulaic.materializers import FormulaMaterializer
 from formulaic.materializers.types import FactorValues, ScopedTerm
 from formulaic.parser.types import Term
 from formulaic.transforms import stateful_transform
@@ -31,7 +31,7 @@
     from formulaic.materializers.types.formula_materializer import EncodedTermStructure
 
 
-class TabmatMaterializer(NarwhalsMaterializer):
+class TabmatMaterializer(FormulaMaterializer):
     """Materializer for pandas input and tabmat output."""
 
     REGISTER_NAME = "tabmat"
@@ -66,6 +66,13 @@ def _init(self):
     def data_context(self):
         return self.__data_context
 
+    @override
+    def _is_categorical(self, values: Any) -> bool:
+        if nw.dependencies.is_narwhals_series(values):
+            if not values.dtype.is_numeric():
+                return True
+        return super()._is_categorical(values)
+
     @override
     def _encode_constant(self, value, metadata, encoder_state, spec, drop_rows):
         series = value * numpy.ones(self.nrows - len(drop_rows))