diff --git a/src/Gui/Document.cpp b/src/Gui/Document.cpp
index 8e1d4bba78..6233922d33 100644
--- a/src/Gui/Document.cpp
+++ b/src/Gui/Document.cpp
@@ -32,6 +32,8 @@
 # include <Inventor/nodes/SoSeparator.h>
 #endif
 
+#include <cctype>
+
 #include <App/AutoTransaction.h>
 #include <App/DocumentObject.h>
 #include <App/DocumentObjectGroup.h>