diff --git a/src/docx/package.py b/src/docx/package.py
index 7ea47e6e1..dd4b198df 100644
--- a/src/docx/package.py
+++ b/src/docx/package.py
@@ -103,7 +103,7 @@ def _next_image_partname(self, ext: str) -> PackURI:
         def image_partname(n: int) -> PackURI:
             return PackURI("/word/media/image%d.%s" % (n, ext))
 
-        used_numbers = [image_part.partname.idx for image_part in self]
+        used_numbers = {image_part.partname.idx for image_part in self}
         for n in range(1, len(self) + 1):
             if n not in used_numbers:
                 return image_partname(n)