GtkTextView and tags

[SkyRat23 aol com]

Hello.  I am trying to set the color of a specific section of a text view and am having problems.  I have a 
tag created with the properties I want and I have two iterators pointing to the boundaries.  However, upon 
execution I get this assertion error from the gtk_text_buffer_apply_tag function:

assertion: 'gtk_text_iter_get_buffer(start) == buffer' failed

Can anyone point out what I have done incorrectly?  A little snippet of code if required would be much 
appreciated.

Thank You.
David Swiston