diff --git a/src/Gui/Document.cpp b/src/Gui/Document.cpp
index f883b80ab6..42c6812944 100644
--- a/src/Gui/Document.cpp
+++ b/src/Gui/Document.cpp
@@ -38,6 +38,8 @@
 # include <Inventor/nodes/SoSeparator.h>
 #endif
 
+#include <cctype>
+
 #include <Base/Console.h>
 #include <Base/Exception.h>
 #include <Base/Matrix.h>