insert comment / undo failing in gtksourceview

Question

I've just stumbled upon a subtle problem with my insert comment and undo functions. Steps to reproduce:

1. open editor
2. insert comment somewhere in the sourcebuffer
3. hit ctrl+z to undo

4. insert another comment, the comment is inserted ok, but the following error shows up in the stdout:

   GtkSourceView-CRITICAL **: modified_changed_handler: assertion `action != NULL' failed

5. insert another comment, which also works

6. try to undo by hitting ctrl+z, undo does not work, and the following error shows up in stdout:

   GtkSourceView-CRITICAL **: gtk_source_undo_manager_undo_impl: assertion `undo_action != NULL' failed

From here on undo does not work.

Here is my insert_comment():

    ...
tbuffer = gtk_text_view_get_buffer (GTK_TEXT_VIEW(txtinput[current_tab]));
gtk_text_buffer_get_selection_bounds (tbuffer, &start, &end);

gtk_text_iter_set_line_offset (&start, 0);
gtk_text_buffer_insert (tbuffer, &start, "// \0", -1);
    ...

and my undo keypress definition:

    case GDK_KEY_z:
        if (event -> state & GDK_CONTROL_MASK)
        {

            GtkSourceBuffer *sbuffer;

            sbuffer = GTK_SOURCE_BUFFER(gtk_text_view_get_buffer(GTK_TEXT_VIEW(txtinput[current_tab])));

            if (gtk_source_buffer_can_undo(sbuffer))
            {
                gtk_source_buffer_undo(sbuffer);
            }

            return TRUE;
        }

        break;

I saw one other person who had this same problem, but no solution was presented. Has anyone else had this problem?

I tried commenting out the "modified-changed" signal connection as a troubleshooting step, but the problem persists.

Any ideas on what I might be doing wrong? Thanks.