QGIS returns "Invalid Layer" message when trying to add PostGIS view?

I'm new to Postgis and QGIS. I created a view defined as

CREATE OR REPLACE VIEW pupr.vstudentmuni AS 
 SELECT m.id AS munid, m.name AS muname, m.geom, s.code, 
    (s.name::text || ' '::text) || s.lastname::text AS fullname
   FROM pupr.municipio m, pupr.student_location l, pupr.student s
  WHERE st_within(l.geom, m.geom) AND s.code = l.code;

in PostgreSQL. Trying to add this view to QGIS returns the following error...

"Invalid layer"..."sql= is an invalid layer and cannot be loaded."

Not sure what the problem is. The table is mentioned in the geommetry_columns table in PostGIS, but I can see a message entry in QGIS that says "No key field for given view". I'm using PostgreSQL 9.2 and QGIS 1.8.0.