--- a/ClassDefinitionChange.st	Tue Aug 27 11:19:08 2019 +0200
+++ b/ClassDefinitionChange.st	Tue Aug 27 11:21:33 2019 +0200
@@ -734,8 +734,8 @@
 
     private == true ifTrue:[^ self].
 
-    parseTree := Parser parseExpression:self source.
-    parseTree isMessage ifFalse:[
+    parseTree := Parser parseExpression:(self source) onError:nil.
+    (parseTree notNil and:[parseTree isMessage]) ifFalse:[
         self error:'bad change source'.
     ].
 
@@ -794,7 +794,7 @@
     |parseTree catIdx poolIdx instVarIdx classVarIdx |
 
     source notNil ifTrue:[
-        parseTree := Parser parseExpression:source.
+        parseTree := Parser parseExpression:source onError:nil.
         (parseTree notNil and:[parseTree isMessage]) ifFalse:[
             self error:'bad change source' mayProceed:true.
             ^ self