diff --git a/clang-tools-extra/clang-tidy/readability/UppercaseLiteralSuffixCheck.cpp b/clang-tools-extra/clang-tidy/readability/UppercaseLiteralSuffixCheck.cpp
index dc2882418d35..badbf2a0d96f 100644
--- a/clang-tools-extra/clang-tidy/readability/UppercaseLiteralSuffixCheck.cpp
+++ b/clang-tools-extra/clang-tidy/readability/UppercaseLiteralSuffixCheck.cpp
@@ -13,6 +13,7 @@
 #include "clang/Lex/Lexer.h"
 #include "llvm/ADT/Optional.h"
 #include "llvm/ADT/SmallString.h"
+#include <cctype>
 
 using namespace clang::ast_matchers;