diff --git a/src/pg/sql/08_interpolation.sql b/src/pg/sql/08_interpolation.sql
index 04f1584..76fad01 100644
--- a/src/pg/sql/08_interpolation.sql
+++ b/src/pg/sql/08_interpolation.sql
@@ -14,9 +14,12 @@ $$
 DECLARE
     gs geometry[];
     vs numeric[];
+    output numeric;
 BEGIN
     EXECUTE 'WITH a AS('||query||') SELECT array_agg(the_geom), array_agg(attrib) FROM a' INTO gs, vs;
-    RETURN QUERY SELECT CDB_SpatialInterpolation(gs, vs, point, method, p1,p2) FROM a;
+    SELECT CDB_SpatialInterpolation(gs, vs, point, method, p1,p2) INTO output FROM a;
+
+    RETURN output;
 END;
 $$
 language plpgsql IMMUTABLE;