diff --git a/lib/core/libwebsockets.c b/lib/core/libwebsockets.c
index 4fd5d561c..21e77efbd 100644
--- a/lib/core/libwebsockets.c
+++ b/lib/core/libwebsockets.c
@@ -867,7 +867,7 @@ lws_snprintf(char *str, size_t size, const char *format, ...)
 	va_list ap;
 	int n;
 
-	if (!size)
+	if (!str || !size)
 		return 0;
 
 	va_start(ap, format);