diff --git a/machine/corpora/parallel_text_corpus.py b/machine/corpora/parallel_text_corpus.py
index 5f17c97..f06a4ec 100644
--- a/machine/corpora/parallel_text_corpus.py
+++ b/machine/corpora/parallel_text_corpus.py
@@ -617,8 +617,8 @@ def count(self, include_empty: bool = True, text_ids: Optional[Iterable[str]] =
             if include_empty:
                 return len(self._df)
             return len(self._df[(self._df[self._source_column] != "") & (self._df[self._target_column] != "")])
-        return len(self._df[self._df[self._source_column].isin(text_ids)]) & (
-            len(self._df[self._target_column].isin(text_ids))
+        return len(self._df[self._df[self._source_column].isin(cast(Sequence[str], text_ids))]) & (
+            len(self._df[self._target_column].isin(cast(Sequence[str], text_ids))
         )
 
     def _get_rows(self, text_ids: Optional[Iterable[str]] = None) -> Generator[ParallelTextRow, None, None]: