diff --git a/test/format-test.cc b/test/format-test.cc
index 70bceee0..d4b54a00 100644
--- a/test/format-test.cc
+++ b/test/format-test.cc
@@ -28,6 +28,7 @@
 #include <cctype>
 #include <cfloat>
 #include <climits>
+#include <cmath>
 #include <cstdarg>
 #include <cstring>
 #include <fstream>
@@ -49,7 +50,6 @@ FILE *FOpen(const char *filename, const char *mode) {
 #include "util.h"
 #include "gtest-extra.h"
 
-#include <math.h>
 #include <stdint.h>
 
 #undef min
@@ -1229,11 +1229,17 @@ TEST(FormatterTest, FormatDouble) {
   EXPECT_EQ("+0000392.6", str(Format("{0:+010.4g}") << 392.65));
 }
 
+TEST(FormatterTest, SignBit) {
+#ifndef signbit
+# define signbit std::signbit
+#endif
+  EXPECT_EQ(1, signbit(-std::numeric_limits<double>::quiet_NaN()));
+}
+
 TEST(FormatterTest, FormatNaN) {
   double nan = std::numeric_limits<double>::quiet_NaN();
   EXPECT_EQ("nan", str(Format("{}") << nan));
   EXPECT_EQ("+nan", str(Format("{:+}") << nan));
-  EXPECT_EQ(1, signbit(-nan));
   EXPECT_EQ("-nan", str(Format("{}") << -nan));
   EXPECT_EQ(" nan", str(Format("{: }") << nan));
   EXPECT_EQ("NAN", str(Format("{:F}") << nan));