Record update problem - empty SET clause

Hi, Hopefully someone can help with what is probably a simple question.

I have a simple model, initially created as a scaffold, and the "update" action does not seem to work. I set the following error:

Mysql::Error: #42000You have an error in your SQL syntax; check the manual that corresponds to your MySQL server version for the right syntax to use near 'WHERE `id` = 9' at line 1: UPDATE layers SET WHERE `id` = 9

Clearly, the SQL is bad since the "SET" clause is empty! Here is the code in the controller:

def update     @layer = Layer.find(params[:id])     if @layer.update_attributes(params[:layer])         flash[:notice] = 'Layer was successfully updated.'         redirect_to :action => 'show', :id => @layer     else         render :action => 'edit'     end end

The request parameters look like this, according to the log:

Parameters: {"commit"=>"Save changes", "id"=>"9", "layer"=>{"name"=>"incidents", "uri"=>"c:/file.txt", "fnameattr"=>"name", "description"=>"Test Map"}}

The database table looks right and the fields seem correct. Can anyone help with this one? Thanks in advance!