--- a/Tools__MethodCategoryList.st	Tue May 08 20:23:32 2018 +0200
+++ b/Tools__MethodCategoryList.st	Wed May 09 14:34:57 2018 +0200
@@ -570,13 +570,17 @@
             or:[ something == #methodCategoriesRemoved
             or:[ something == #methodCategoryRenamed ]]]]) ifTrue:[
                 cls := (something == #classOrganization) ifTrue:aParameter ifFalse:[aParameter first].
-                (classes includesIdentical:cls) ifTrue:[
-                    listValid == true ifTrue:[ self invalidateList ].
+                (classes includesIdentical:nil) ifTrue:[
+                    self invalidateList
                 ] ifFalse:[
-                    (classes contains:[:aClass | aClass name = cls name]) ifTrue:[
+                    (classes includesIdentical:cls) ifTrue:[
                         listValid == true ifTrue:[ self invalidateList ].
-                        "/ self error:'obsolete class: should not happen'.
-                    ]
+                    ] ifFalse:[
+                        (classes contains:[:aClass | aClass notNil and:[aClass name = cls name]]) ifTrue:[
+                            listValid == true ifTrue:[ self invalidateList ].
+                            "/ self error:'obsolete class: should not happen'.
+                        ]
+                    ].
                 ].
                 ^ self.
             ].