diff --git a/expert/concept.py b/expert/concept.py
index 5b81035..dc238b2 100644
--- a/expert/concept.py
+++ b/expert/concept.py
@@ -148,10 +148,11 @@ class FullQuery(BaseQuery):
                 if not r1 and not r2:
                     r1 = cat.apply(criteria)    # search only for type
                 x, uids = weightedUnion(r1, r2) #result = r1.union(r2)
-                for r, score in uids.items():
-                    obj = intids.getObject(r)
-                    result.add(obj)
-                    scores[obj] = score
+                if uids:
+                    for r, score in uids.items():
+                        obj = intids.getObject(r)
+                        result.add(obj)
+                        scores[obj] = score
         if rc is not None:
             if result:
                 result = result.intersection(rc)