diff --git a/src/pydiverse/transform/_internal/ops/op.py b/src/pydiverse/transform/_internal/ops/op.py
index c0b4e6b..acf491b 100644
--- a/src/pydiverse/transform/_internal/ops/op.py
+++ b/src/pydiverse/transform/_internal/ops/op.py
@@ -76,6 +76,6 @@ def return_type(self, signature: Sequence[Dtype]) -> Dtype:
         if match is None:
             raise TypeError(
                 f"operator `{self.name}` cannot be called with arguments of type "
-                f"{", ".join(str(t) for t in signature)}"
+                f'{", ".join(str(t) for t in signature)}'
             )
         return match[1]
diff --git a/src/pydiverse/transform/_internal/tree/col_expr.py b/src/pydiverse/transform/_internal/tree/col_expr.py
index b3bc941..eb7cb4e 100644
--- a/src/pydiverse/transform/_internal/tree/col_expr.py
+++ b/src/pydiverse/transform/_internal/tree/col_expr.py
@@ -1104,7 +1104,7 @@ def dtype(self):
             )
         ):
             raise TypeError(
-                f"incompatible types `{", ".join(val_types)}` in case expression."
+                f'incompatible types `{", ".join(val_types)}` in case expression.'
             )
 
         if any(cond.dtype() is None for cond, _ in self.cases):