diff --git a/src/proto.h b/src/proto.h
index 9d1e134..50802ce 100644
--- a/src/proto.h
+++ b/src/proto.h
@@ -215,6 +215,7 @@
 # ifdef FEAT_EVAL
 // include vim9.h here, the types defined there are used by function arguments.
 #  include "vim9.h"
+#  include "vim9class.pro"
 #  include "vim9cmds.pro"
 #  include "vim9compile.pro"
 #  include "vim9execute.pro"