diff --git a/memcheck/mc_main.c b/memcheck/mc_main.c
index 0c09b18..06546d2 100644
--- a/memcheck/mc_main.c
+++ b/memcheck/mc_main.c
@@ -2196,7 +2196,7 @@
 {
    tl_assert(sizeof(UWord) == sizeof(SizeT));
    if (0)
-      VG_(printf)("helperc_MAKE_STACK_UNINIT %p %d\n", base, len );
+      VG_(printf)("helperc_MAKE_STACK_UNINIT %p %lu\n", base, len );
 
 #  if 0
    /* Really slow version */