GtkTextTag usage

[Nick Watts <fatalglory gmail com>]

I'm trying to implement syntax highlighting in my text widget.  Here is my
code:

static void create_tags(GtkTextBuffer *buffer)
       {
            gtk_text_buffer_create_tag(buffer, "doctype", "background",
"grey", "editable", FALSE, NULL);
       }

static void text_changed(GtkTextBuffer *buffer, gpointer data)
       {
            changed = 1;
            GtkTextIter start;
            GtkTextIter end;
            gtk_text_buffer_get_iter_at_offset(buffer, &start, 0);
            gtk_text_buffer_get_iter_at_offset(buffer, &end, -1);
            gtk_text_buffer_remove_all_tags(buffer, &start, &end);
            gtk_text_buffer_get_iter_at_line_offset(buffer, &start, 0, 0);
            gtk_text_buffer_get_iter_at_line_offset(buffer, &end, 1, 0);
            gtk_text_buffer_apply_tag_by_name(buffer, "doctype", &start,
&end);
       }

The text_changed function is connected to the "changed" signal of the
buffer, and i can apply the tag fine, but when I start the application I get
this error:

(application.exe:2792): GtkWarning **: Unknown tag 'doctype'

It doesn't seem to have any affect so right now I'm just letting sleeping
dogs lie, but does anyone know what's wrong?