diff --git a/include/spdlog/details/os-inl.h b/include/spdlog/details/os-inl.h
index 001f34ae..b7817184 100644
--- a/include/spdlog/details/os-inl.h
+++ b/include/spdlog/details/os-inl.h
@@ -511,7 +511,7 @@ SPDLOG_INLINE void utf8_to_wstrbuf(string_view_t str, wmemory_buf_t &target)
         result_size = ::MultiByteToWideChar(CP_UTF8, MB_ERR_INVALID_CHARS, str.data(), str_size, target.data(), result_size);
         if (result_size  > 0)
         {
-            assert result_size == target.size();
+            assert(result_size == target.size());
             return;
         }
     }