diff --git a/configure.ac b/configure.ac
index 796e5cf3b..c85da0c29 100644
--- a/configure.ac
+++ b/configure.ac
@@ -185,7 +185,7 @@ AC_CACHE_CHECK([Size of long int],
 [iperf3_cv_sizeof_long],
 AC_RUN_IFELSE(
   [AC_LANG_PROGRAM([[#include <stdio.h>]],
-                   [[fprintf(stderr, "%d", sizeof(long)); return(0)]])],
+                   [[fprintf(stderr, "%zd", sizeof(long)); return(0)]])],
   [iperf3_cv_sizeof_long=`./conftest$EXEEXT 2>&1`],
   [AC_MSG_ERROR(Failed to compile and run size of long test)]))
 AC_DEFINE_UNQUOTED([SIZE_OF_LONG_INT], $iperf3_cv_sizeof_long, [Size of long int.])