diff --git a/mlir/lib/TableGen/Format.cpp b/mlir/lib/TableGen/Format.cpp
index 6dd260ac9e77..967d51a61f70 100644
--- a/mlir/lib/TableGen/Format.cpp
+++ b/mlir/lib/TableGen/Format.cpp
@@ -22,6 +22,7 @@
 //===----------------------------------------------------------------------===//
 
 #include "mlir/TableGen/Format.h"
+#include <cctype>
 
 using namespace mlir;
 using namespace mlir::tblgen;