gtksourceview gtk_source_view_get_type return 0

I'm not sure if this the correct forum to post, so sorry if it isn't.

I want to use glib api 'g_type_check_instance_is_a window' to test if a widget 
is a gtksourceview like this
   g_type_check_instance_is_a (window , gtk_source_view_get_type())

but the function 'gtk_source_view_get_type' returns 0. Other gtk widgets (eg. 
menu_bar_get_type) work properly

Does anyone know how to make 'gtk_source_view_get_type' work?


thanks