diff --git a/lib/Frontend/LayoutOverrideSource.cpp b/lib/Frontend/LayoutOverrideSource.cpp
index eb7865e..e023250 100644
--- a/lib/Frontend/LayoutOverrideSource.cpp
+++ b/lib/Frontend/LayoutOverrideSource.cpp
@@ -9,6 +9,7 @@
 #include "clang/Frontend/LayoutOverrideSource.h"
 #include "clang/AST/Decl.h"
 #include "llvm/Support/raw_ostream.h"
+#include <cctype>
 #include <fstream>
 #include <string>