diff --git a/cpp/httpclientpool.cc b/cpp/httpclientpool.cc
index 46e83c8..47a7762 100644
--- a/cpp/httpclientpool.cc
+++ b/cpp/httpclientpool.cc
@@ -441,7 +441,7 @@ namespace f8
 
     static void locking_function(int mode, int n, const char *file, int line)
     {
-        if (n != f8::HttpClientPool::Instance()->Impl()->mutex_buf_size) {
+        if (n != (int)f8::HttpClientPool::Instance()->Impl()->mutex_buf_size) {
             abort();
         }
         pthread_mutex_t* mutex = &f8::HttpClientPool::Instance()->Impl()->mutex_buf[n];