diff --git a/lib/checksizeof.cpp b/lib/checksizeof.cpp
index 8df66566b..ac33b9a59 100644
--- a/lib/checksizeof.cpp
+++ b/lib/checksizeof.cpp
@@ -21,6 +21,7 @@
 #include "checksizeof.h"
 #include "symboldatabase.h"
 #include <algorithm>
+#include <cctype>
 //---------------------------------------------------------------------------
 
 // Register this check class (by creating a static instance of it)