diff --git a/python/tempo/tsdf.py b/python/tempo/tsdf.py
index c8e9ce30..6cd6ab54 100644
--- a/python/tempo/tsdf.py
+++ b/python/tempo/tsdf.py
@@ -110,7 +110,7 @@ def __validate_ts_string(ts_text: str) -> None:
 
     @staticmethod
     def __validated_column(df: DataFrame, colname: str) -> str:
-        if colname is not str:
+        if type(colname) != str:
             raise TypeError(
                 f"Column names must be of type str; found {type(colname)} instead!"
             )
@@ -122,20 +122,19 @@ def __validated_columns(
         self, df: DataFrame, colnames: Optional[Union[str, List[str]]]
     ) -> List[str]:
         # if provided a string, treat it as a single column
-        valid_colnames: List[str] = []
-        if colnames is str:
-            valid_colnames = [str(colnames)]
+        if type(colnames) == str:
+            colnames = [colnames]
         # otherwise we really should have a list or None
         elif colnames is None:
-            valid_colnames = []
-        elif colnames is not list:
+            colnames = []
+        elif type(colnames) != list:
             raise TypeError(
                 f"Columns must be of type list, str, or None; found {type(colnames)} instead!"
             )
         # validate each column
-        for col in valid_colnames:
+        for col in colnames:
             self.__validated_column(df, col)
-        return valid_colnames
+        return colnames
 
     def __checkPartitionCols(self, tsdf_right: "TSDF") -> None:
         for left_col, right_col in zip(self.partitionCols, tsdf_right.partitionCols):