diff --git a/prism/src/prism/ProbModel.java b/prism/src/prism/ProbModel.java
index 7e32735a..842b320f 100644
--- a/prism/src/prism/ProbModel.java
+++ b/prism/src/prism/ProbModel.java
@@ -870,18 +870,21 @@ public class ProbModel implements Model
 		return index;
 	}
 
-	// clear up (deref all dds, dd vars)
-
+	/**
+	 * Clear the model (deref all DDs and DD variables)
+	 */
 	public void clear()
 	{
-		for (int i = 0; i < numVars; i++) {
-			varDDRowVars[i].derefAll();
-			varDDColVars[i].derefAll();
-		}
-		for (int i = 0; i < numModules; i++) {
-			moduleDDRowVars[i].derefAll();
-			moduleDDColVars[i].derefAll();
-		}
+		if (varDDRowVars != null)
+			JDDVars.derefAllArray(varDDRowVars);
+		if (varDDColVars != null)
+			JDDVars.derefAllArray(varDDColVars);
+
+		if (moduleDDRowVars != null)
+			JDDVars.derefAllArray(moduleDDRowVars);
+		if (moduleDDColVars != null)
+			JDDVars.derefAllArray(moduleDDColVars);
+
 		allDDRowVars.derefAll();
 		allDDColVars.derefAll();
 		JDD.Deref(trans);
diff --git a/prism/src/prism/StateModelChecker.java b/prism/src/prism/StateModelChecker.java
index 816b3f54..c2bf5e2b 100644
--- a/prism/src/prism/StateModelChecker.java
+++ b/prism/src/prism/StateModelChecker.java
@@ -151,9 +151,9 @@ public class StateModelChecker implements ModelChecker
 		                      null,             // moduleNames
 		                      null,             // moduleRowVars
 		                      null,             // moduleColVars
-		                      0,                // numVars
+		                      varDDRowVars.length, // numVars
 		                      varList,          // varList
-		                      varDDRowVars,     // varDDRowVars
+		                      JDDVars.copyArray(varDDRowVars), // varDDRowVars
 		                      null,             // varDDColVars
 		                      constantValues    // constantValues
 		                     );