diff --git a/format-test.cc b/format-test.cc
index c851f184..9da629a5 100644
--- a/format-test.cc
+++ b/format-test.cc
@@ -268,9 +268,10 @@ TEST(UtilTest, StrError) {
   EXPECT_STREQ("", message);
   result = StrError(error_code, message = buffer, BUFFER_SIZE);
   EXPECT_EQ(0, result);
-  EXPECT_GE(BUFFER_SIZE - 1u, std::strlen(message));
+  std::size_t message_size = std::strlen(message);
+  EXPECT_GE(BUFFER_SIZE - 1u, message_size);
   EXPECT_STREQ(strerror(error_code), message);
-  result = StrError(error_code, message = buffer, std::strlen(message));
+  result = StrError(error_code, message = buffer, message_size);
   EXPECT_EQ(ERANGE, result);
 }