diff --git a/lib/SPIRV/libSPIRV/SPIRVStream.h b/lib/SPIRV/libSPIRV/SPIRVStream.h
index d7c26614af..bc2beca4cc 100644
--- a/lib/SPIRV/libSPIRV/SPIRVStream.h
+++ b/lib/SPIRV/libSPIRV/SPIRVStream.h
@@ -44,6 +44,7 @@
 #include "SPIRVExtInst.h"
 #include "SPIRVModule.h"
 #include <algorithm>
+#include <cctype>
 #include <cstdint>
 #include <iostream>
 #include <iterator>