diff --git a/format.cc b/format.cc
index c771d33c..5f07c548 100644
--- a/format.cc
+++ b/format.cc
@@ -30,6 +30,7 @@
 #include <stdint.h>
 
 #include <cassert>
+#include <cctype>
 #include <climits>
 #include <cstring>
 #include <algorithm>