title: assign_vertex_id error
body: <p>I am trying to test if pgrouting works fine or not. To do so I have created a table containing the following attributes. </p>
  
  <pre>
    Columns:
   gid      |
   length   |
   the_geom |
   source   |
   target
  </pre>
  
  <p>Now my problem is that when I try to execute the assign_vertex_id function it is giving me the following error;  </p>
  
  <pre>
  PL/pgSQL function "assign_vertex_id" line 15 at EXECUTE statement
  ERROR:  query string argument of EXECUTE is null
  CONTEXT:  PL/pgSQL function "assign_vertex_id" line 32 at EXECUTE statement
  
  ********** Error **********
  
  ERROR: query string argument of EXECUTE is null
  SQL state: 22004
  
  
  Context: PL/pgSQL function "assign_vertex_id" line 32 at EXECUTE statement
  </pre>
  
  <p>Any suggestions what does this mean?</p>