diff --git a/find_dupes.py b/find_dupes.py
index ce6e8f3..19bc894 100644
--- a/find_dupes.py
+++ b/find_dupes.py
@@ -93,9 +93,10 @@ def main():
     n = len(names)
     print(f"Pairwise compare {n}x{n} (numpy hamming) ...")
     dmat = np.array([data[f][0] for f in names], dtype=np.uint64)
-    # hamming distances via xor + popcount
-    x = dmat[:, None] ^ dmat[None, :]
-    ham = np.zeros_like(x)
+    # hamming distances via xor + popcount (work in int64; numpy has no
+    # sane uint64 shift-add promotion)
+    x = (dmat[:, None] ^ dmat[None, :]).astype(np.int64)
+    ham = np.zeros(x.shape, dtype=np.int16)
     for b in range(64):
         ham += ((x >> b) & 1).astype(np.int16)