diff --git a/Lib/test/test_long.py b/Lib/test/test_long.py
index 569ab15820e302..9cd74c88a3ccca 100644
--- a/Lib/test/test_long.py
+++ b/Lib/test/test_long.py
@@ -1601,5 +1601,12 @@ def test_square(self):
                 self.assertEqual(n**2,
                     (1 << (2 * bitlen)) - (1 << (bitlen + 1)) + 1)
 
+    def test_long_add_overallocate(self):
+        # see gh-100688
+        x = (MASK//2) * (MASK+1)
+        x2 = (MASK//2 + 1) * (MASK+1)
+        z = x + x2
+        self.assertEqual(x + x2, MASK * (MASK + 1))
+
 if __name__ == "__main__":
     unittest.main()