--- a/js/src/xsum/xsum.cpp
+++ b/js/src/xsum/xsum.cpp
@@ -266,7 +266,7 @@
 
   /* At this point, sacc->chunk[u] must be non-zero */
 
-  if (xsum_debug) printf("u: %d, sacc->chunk[u]: %ld", u, sacc->chunk[u]);
+  if (xsum_debug) printf("u: %d, sacc->chunk[u]: %lld", u, sacc->chunk[u]);
 
   /* Carry propagate, starting at the low-order chunks.  Note that the
      loop limit of u may be increased inside the loop. */