diff --git a/pandas/io/formats/format.py b/pandas/io/formats/format.py
index 15f21814b072d..ad62c56a337b6 100644
--- a/pandas/io/formats/format.py
+++ b/pandas/io/formats/format.py
@@ -868,6 +868,8 @@ def _join_multiline(self, *args) -> str:
             np.array([self.adj.len(x) for x in col]).max() if len(col) > 0 else 0
             for col in strcols
         ]
+
+        assert lwidth is not None
         col_bins = _binify(col_widths, lwidth)
         nbins = len(col_bins)
 
@@ -1890,7 +1892,7 @@ def set_eng_float_format(accuracy: int = 3, use_eng_prefix: bool = False) -> Non
     set_option("display.column_space", max(12, accuracy + 9))
 
 
-def _binify(cols: List[np.int32], line_width: Union[np.int32, int]) -> List[int]:
+def _binify(cols: List[int], line_width: int) -> List[int]:
     adjoin_width = 1
     bins = []
     curr_width = 0