diff --git a/AlgebraicDataflowArchitectureModel/src/models/algebra/Term.java b/AlgebraicDataflowArchitectureModel/src/models/algebra/Term.java
index 2a22051..d259620 100644
--- a/AlgebraicDataflowArchitectureModel/src/models/algebra/Term.java
+++ b/AlgebraicDataflowArchitectureModel/src/models/algebra/Term.java
@@ -106,6 +106,7 @@
 		pos = (Position) pos.clone();
 		int i = pos.removeHeadOrder();
 		if (i >= children.size()) return null;
+		if (children.get(i) == null) return null;
 		return children.get(i).getSubTerm(pos);
 	}
 	
@@ -143,7 +144,11 @@
 			if (children.size() != anotherTerm.children.size()) return null;
 			Term unifiedTerm = new Term(symbol);
 			for (int i = 0; i < children.size(); i++) {
-				unifiedTerm.addChild(children.get(i).unify(anotherTerm.children.get(i)));
+				if (children.get(i) != null) {
+					unifiedTerm.addChild(children.get(i).unify(anotherTerm.children.get(i)));
+				} else {
+					unifiedTerm.addChild(anotherTerm.children.get(i));
+				}
 			}
 			return unifiedTerm;
 		} else {