--- a/src/main/scala/SparqlToSql.scala	Sun Feb 28 15:04:42 2010 -0500
+++ b/src/main/scala/SparqlToSql.scala	Sun Feb 28 15:05:33 2010 -0500
@@ -242,7 +242,10 @@
 	val objattr = sql.RelVarAttr(relvar, attr)
 	val state_postSubj = s match {
 	  case sparql.TermUri(u) => uriConstraint(stateP, sql.RelVarAttr(relvar, db.relationdescs(rel).primarykey.get), parseObjectURI(u), true)
-	  case sparql.TermVar(v) => varConstraint(stateP, relvar, db.relationdescs(rel).primarykey, v, db, rel)
+	  case sparql.TermVar(v) => try { varConstraint(stateP, relvar, db.relationdescs(rel).primarykey, v, db, rel) } catch {
+	  case e:java.util.NoSuchElementException =>
+	    throw new Exception("error processing { " + s + " " + p + " " + o + " } :db.relationdescs(" + rel + ") not found in " + db)
+	  }
 	  case _                 => error("illegal SPARQL subject: " + s)
 	}
 	val state_subjJoin = R2RState(state_postSubj.joins + sql.InnerJoin(sql.AliasedResource(rel,relvar), None), state_postSubj.varmap, state_postSubj.exprs)