diff --git a/Python/hamt.c b/Python/hamt.c
index 562f777..71a5ec8 100644
--- a/Python/hamt.c
+++ b/Python/hamt.c
@@ -2003,7 +2003,7 @@
             goto error;
         }
 
-        if (_hamt_dump_format(writer, "%d::\n", i)) {
+        if (_hamt_dump_format(writer, "%zd::\n", i)) {
             goto error;
         }