diff --git a/AlgebraicDataflowArchitectureModel/src/models/algebra/Term.java b/AlgebraicDataflowArchitectureModel/src/models/algebra/Term.java
index e78c106..cd3d92d 100644
--- a/AlgebraicDataflowArchitectureModel/src/models/algebra/Term.java
+++ b/AlgebraicDataflowArchitectureModel/src/models/algebra/Term.java
@@ -73,11 +73,13 @@
 			subTerms.put(new Position(), (T) this);
 		}
 		for (int i = 0; i < children.size(); i++) {
-			HashMap<Position, T> terms = children.get(i).getSubTerms(clazz);
-			for (Entry<Position, T> term: terms.entrySet()) {
-				Position pos = term.getKey();
-				pos.addHeadOrder(i);
-				subTerms.put(pos, term.getValue());
+			if (children.get(i) != null) {
+				HashMap<Position, T> terms = children.get(i).getSubTerms(clazz);
+				for (Entry<Position, T> term: terms.entrySet()) {
+					Position pos = term.getKey();
+					pos.addHeadOrder(i);
+					subTerms.put(pos, term.getValue());
+				}
 			}
 		}
 		return subTerms;
@@ -117,6 +119,8 @@
 	@Override
 	public Expression unify(Expression another) {
 		if (another instanceof Variable) return (Expression) this.clone();
+		if (this instanceof Constant) return (Expression) this.clone();
+		if (another instanceof Constant) return (Expression) another.clone();
 		if (another instanceof Term) {
 			Term anotherTerm = (Term) another;
 			if (!symbol.equals(anotherTerm.symbol)) return null;
@@ -232,7 +236,11 @@
 	public Object clone() {
 		Term newTerm = new Term(symbol);
 		for (Expression e: children) {
-			newTerm.addChild((Expression) e.clone());
+			if (e != null) {
+				newTerm.addChild((Expression) e.clone());
+			} else {
+				newTerm.addChild(null);
+			}
 		}
 		newTerm.type = type;
 		return newTerm;
@@ -247,7 +255,11 @@
 			String delimiter = "";
 			for (int i = 1; i < children.size(); i++) {
 				Expression e = children.get(i);
-				exp += (delimiter + e.toString());
+				if (e != null) {
+					exp += (delimiter + e.toString());
+				} else {
+					exp += (delimiter + "null");
+				}
 				delimiter = ",";
 			}
 			return exp + ")";