diff --git a/configure.in b/configure.in
index 0cf8a7d538..36f5501d0a 100644
--- a/configure.in
+++ b/configure.in
@@ -200,7 +200,7 @@ AC_CACHE_CHECK([whether memset(0) sets pointers to NULL], tor_cv_null_is_zero,
 [AC_RUN_IFELSE([AC_LANG_SOURCE(
 [[#include <stdlib.h>
 int main () { char *p1,*p2; p1=NULL; memset(&p2,0,sizeof(p2));
-return memcmp(&p1,&p2,sizeof(char*)); }]])],
+return memcmp(&p1,&p2,sizeof(char*))?1:0; }]])],
        [tor_cv_null_is_zero=yes],
        [tor_cv_null_is_zero=no],
        [tor_cv_null_is_zero=cross])])