diff --git a/gzlib.c b/gzlib.c
index baa3e79d4..4d6692d60 100644
--- a/gzlib.c
+++ b/gzlib.c
@@ -94,7 +94,7 @@ local gzFile gz_open(const void *path, int fd, const char *mode) {
 #endif
 
     /* check input */
-    if (path == NULL)
+    if (path == NULL || mode == NULL)
         return NULL;
 
     /* allocate gzFile structure to return */