diff --git a/prism/src/prism/Prism.java b/prism/src/prism/Prism.java
index a72c3210..e0db21b0 100644
--- a/prism/src/prism/Prism.java
+++ b/prism/src/prism/Prism.java
@@ -1430,6 +1430,8 @@ public class Prism implements PrismSettingsListener
 	 */
 	public void setPRISMModelConstants(Values definedMFConstants) throws PrismLangException
 	{
+		if (currentDefinedMFConstants == null && definedMFConstants == null)
+			return;
 		if (currentDefinedMFConstants != null && currentDefinedMFConstants.equals(definedMFConstants))
 			return;