--- a/Model.st	Wed Nov 09 13:09:45 2016 +0100
+++ b/Model.st	Fri Nov 11 17:21:32 2016 +0100
@@ -213,16 +213,19 @@
         deps := dependents.
         deps notNil ifTrue:[
             deps isCollection ifTrue:[
-                deps remove:anObject ifAbsent:[].
-                (sz := deps size) == 0 ifTrue:[
-                    dependents := nil
-                ] ifFalse:[
-                    sz == 1 ifTrue:[
-                        (dep := deps first) isCollection ifFalse:[
-                            dependents := dep
+                dep := deps remove:anObject ifAbsent:[].
+                "if dep is nil, nothing has changed"
+                dep notNil ifTrue:[
+                    (sz := deps size) == 0 ifTrue:[
+                        dependents := nil
+                    ] ifFalse:[
+                        sz == 1 ifTrue:[
+                            (dep := deps first) isCollection ifFalse:[
+                                dependents := dep
+                            ]
                         ]
-                    ]
-                ]
+                    ].
+                ].
             ] ifFalse:[
                 deps == anObject ifTrue:[
                     dependents := nil