1 files changed, 3 insertions, 1 deletions

diff --git a/src/uk/ac/ox/cs/pagoda/query/rollup/QueryGraph.java b/src/uk/ac/ox/cs/pagoda/query/rollup/QueryGraph.java
index 02697b8..9b4ce01 100644
--- a/src/uk/ac/ox/cs/pagoda/query/rollup/QueryGraph.java
+++ b/src/uk/ac/ox/cs/pagoda/query/rollup/QueryGraph.java
@@ -184,7 +184,9 @@ public class QueryGraph {
 
         private void rollupEdge(Term u, OWLObjectPropertyExpression op, Term v, boolean inverse) {
                 if (existVars.contains(v)) {
-                        concepts.add(u, factory.getOWLObjectSomeValuesFrom(op, factory.getOWLObjectIntersectionOf(concepts.get(v))));
+                        Set<OWLClassExpression> exps = concepts.get(v);
+                        if (exps == null) exps = new HashSet<OWLClassExpression>();
+                        concepts.add(u, factory.getOWLObjectSomeValuesFrom(op, factory.getOWLObjectIntersectionOf(exps)));
                 }
                 else {
                         OWLIndividual obj = getOWLIndividual(v);