row_reference -> path -> iter not working

[ginxd btopenworld com]

I have used gtk_tree_selection_set_select_function to be notified when a selection of a row takes place.   
When I get called back into this function, I use gtk_tree_row_reference_new(model,path) to set a reference to 
that row.  

When I click on a button I want to use that reference and iterate to the next item on the tree/list (which is 
flat, i.e. has no depth to it).  When I use gtk_tree_row_reference_get_path(row_ref) I am given the path, 
when I use this path to get the iterator (with gtk_tree_model_get_iter) I then use 

  valid = gtk_tree_model_iter_next(iter).

however, valid is FALSE.  How is this possible if I have NOT deleted/modified the item?

Any ideas?

Regards,

Martyn Russell