diff --git a/dtypes.py b/dtypes.py
index 1473909..2be6a50 100644
--- a/dtypes.py
+++ b/dtypes.py
@@ -101,7 +101,7 @@ def get_or_register_dtype(
 
         return dtype
 
-    def dtype_to_ctype(self, dtype: DTypeLike) -> str:
+    def dtype_to_ctype(self, dtype: DTypeLike | None) -> str:
         if dtype is None:
             raise ValueError("dtype may not be None")