diff --git a/src/json.hpp b/src/json.hpp
index 1cbcedf6..0e8eece3 100644
--- a/src/json.hpp
+++ b/src/json.hpp
@@ -32,6 +32,7 @@ SOFTWARE.
 #include <algorithm>
 #include <array>
 #include <cassert>
+#include <cctype>
 #include <ciso646>
 #include <cmath>
 #include <cstddef>
diff --git a/src/json.hpp.re2c b/src/json.hpp.re2c
index 2dabc031..757f8859 100644
--- a/src/json.hpp.re2c
+++ b/src/json.hpp.re2c
@@ -32,6 +32,7 @@ SOFTWARE.
 #include <algorithm>
 #include <array>
 #include <cassert>
+#include <cctype>
 #include <ciso646>
 #include <cmath>
 #include <cstddef>