diff --git a/src/netif/ppp/polarssl/md5.c b/src/netif/ppp/polarssl/md5.c
index 50aa911f..9d0dae0a 100644
--- a/src/netif/ppp/polarssl/md5.c
+++ b/src/netif/ppp/polarssl/md5.c
@@ -43,6 +43,8 @@
 
 #include "netif/ppp/polarssl/md5.h"
 
+#include <string.h>
+
 /*
  * 32-bit integer manipulation macros (little endian)
  */
diff --git a/src/netif/ppp/utils.c b/src/netif/ppp/utils.c
index ecd305a0..4f14b8d9 100644
--- a/src/netif/ppp/utils.c
+++ b/src/netif/ppp/utils.c
@@ -210,7 +210,7 @@ int ppp_vslprintf(char *buf, int buflen, const char *fmt, va_list args) {
 		val = va_arg(args, long);
 		if ((long)val < 0) {
 		    neg = 1;
-		    val = (unsigned long)-val;
+		    val = (unsigned long)-(long)val;
 		}
 		base = 10;
 		break;