diff --git a/plotpy/items/image/mixin.py b/plotpy/items/image/mixin.py
index 43ba5e7..d7f8afd 100644
--- a/plotpy/items/image/mixin.py
+++ b/plotpy/items/image/mixin.py
@@ -573,7 +573,7 @@ def mask_rectangular_area(
         if inside:
             self.data[iy0:iy1, ix0:ix1] = np.ma.masked
         else:
-            indexes = np.ones(self.data.shape, dtype=np.bool_)
+            indexes = np.ones(self.data.shape, dtype=bool)
             indexes[iy0:iy1, ix0:ix1] = False
             self.data[indexes] = np.ma.masked
         if trace: