diff --git a/src/gui/widgets/viewport.hpp b/src/gui/widgets/viewport.hpp
index d469d117a9a..f6df8037371 100644
--- a/src/gui/widgets/viewport.hpp
+++ b/src/gui/widgets/viewport.hpp
@@ -22,7 +22,7 @@
 namespace gui2 {
 
 namespace implementation {
-class tbuilder_viewport;
+struct tbuilder_viewport;
 } // namespace implementation
 
 class tgrid;