diff --git a/base/file_test.cpp b/base/file_test.cpp
index cbb2751..b138094 100644
--- a/base/file_test.cpp
+++ b/base/file_test.cpp
@@ -66,7 +66,7 @@
       << strerror(errno);
   struct stat sb;
   ASSERT_EQ(0, stat(tf.filename, &sb));
-  ASSERT_EQ(0660U, (sb.st_mode & ~S_IFMT));
+  ASSERT_EQ(0660U, static_cast<unsigned int>(sb.st_mode & ~S_IFMT));
   ASSERT_EQ(getuid(), sb.st_uid);
   ASSERT_EQ(getgid(), sb.st_gid);
   std::string s;