diff --git a/src/FactUnion.cpp b/src/FactUnion.cpp
index f83d1a20b..77a37fbad 100644
--- a/src/FactUnion.cpp
+++ b/src/FactUnion.cpp
@@ -245,6 +245,8 @@ FactUnion::join_var_facts(const vector<const Fact*>& facts, const vector<const V
 			if (fu == 0) {
 				fu = dynamic_cast<FactUnion*>(exist_fact->clone());
 			} else {
+				// hack: both facts have to be the property of the same variable to be able to merge
+				fu->set_var(exist_fact->get_var()); 
 				fu->join(*exist_fact);
 			}
 		}
diff --git a/src/FactUnion.h b/src/FactUnion.h
index 387563bb3..1ff378403 100644
--- a/src/FactUnion.h
+++ b/src/FactUnion.h
@@ -57,6 +57,7 @@ class FactUnion : public Fact
 	virtual ~FactUnion(void) {}; 
 
 	virtual const Variable* get_var(void) const { return var;};
+	void set_var(const Variable* v) { var = v;}
 	const Type* get_last_written_type(void) const;
 	int   get_last_written_fid(void) const { return last_written_fid; };
 	static bool is_field_readable(const Variable* v, int fid, const vector<const Fact*>& facts);