diff --git a/src/nghttp.cc b/src/nghttp.cc
index c90b5ad2..2ee49fd0 100644
--- a/src/nghttp.cc
+++ b/src/nghttp.cc
@@ -1467,7 +1467,7 @@ void print_help(std::ostream& out)
 int main(int argc, char **argv)
 {
   while(1) {
-    int flag;
+    int flag = 0;
     static option long_options[] = {
       {"verbose", no_argument, nullptr, 'v'},
       {"null-out", no_argument, nullptr, 'n'},