diff --git a/libparistraceroute/ping/ping.c b/libparistraceroute/ping/ping.c
index 023c7d9..2c891fe 100755
--- a/libparistraceroute/ping/ping.c
+++ b/libparistraceroute/ping/ping.c
@@ -451,7 +451,7 @@ int main(int argc, char ** argv)
                desired_size = (size_t) packet_size[0],
                minimal_size;
         if (!check_packet_size(is_icmp, headers_size, desired_size, &minimal_size)) {
-            fprintf(stderr, "Packet size (%lu) too small (try a value >= %lu)\n", desired_size, minimal_size);
+            fprintf(stderr, "Packet size (%lu) too small (try a value >= %lu)\n", (unsigned long)desired_size, (unsigned long)minimal_size);
             goto ERR_INVALID_PACKET_SIZE;
         }
         probe_payload_resize(probe, desired_size - headers_size);