diff --git a/onnxruntime/core/flatbuffers/flatbuffers_utils.h b/onnxruntime/core/flatbuffers/flatbuffers_utils.h
index 4bbcd02c50..58064c63e3 100644
--- a/onnxruntime/core/flatbuffers/flatbuffers_utils.h
+++ b/onnxruntime/core/flatbuffers/flatbuffers_utils.h
@@ -3,6 +3,7 @@
 
 #pragma once
 
+#include <cctype>
 #include <unordered_map>
 #include <core/common/status.h>