diff --git a/prism/src/pta/DBM.java b/prism/src/pta/DBM.java
index bf7381be..62eee2c8 100644
--- a/prism/src/pta/DBM.java
+++ b/prism/src/pta/DBM.java
@@ -219,6 +219,8 @@ public class DBM extends Zone
 	public void cClosure(int c)
 	{
 		int i, j, n;
+		if (isEmpty())
+			return;
 		n = pta.numClocks;
 		for (i = 0; i < n + 1; i++) {
 			for (j = 0; j < n + 1; j++) {