diff --git a/lib/roken/hex.c b/lib/roken/hex.c
index b5ce01e36..68c18a4eb 100644
--- a/lib/roken/hex.c
+++ b/lib/roken/hex.c
@@ -60,8 +60,10 @@ hex_encode(const void *data, size_t size, char **str)
     char *p;
 
     /* check for overflow */
-    if (size * 2 < size)
+    if (size * 2 < size) {
+        *str = NULL;
 	return -1;
+    }
 
     p = malloc(size * 2 + 1);
     if (p == NULL) {