diff --git a/format_test.cc b/format_test.cc
index 67a01c76..232371bd 100644
--- a/format_test.cc
+++ b/format_test.cc
@@ -25,6 +25,7 @@
  SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
  */
 
+#include <cctype>
 #include <cfloat>
 #include <climits>
 #include <cstring>