--- a/SnapShotImageMemory.st	Wed Sep 05 19:02:21 2001 +0200
+++ b/SnapShotImageMemory.st	Wed Sep 05 19:06:05 2001 +0200
@@ -1094,14 +1094,16 @@
 sourceFilename
     "return the sourcefilename if source is extern; nil otherwise"
 
-    |source|
+    |sourcePtr sourceRef source|
 
     self isMethodOrLazyMethod ifTrue:[
         self sourcePosition notNil ifTrue:[
-            source := self at:(Method instVarOffsetOf:'source').
-            source isString ifTrue:[
-                source := memory printStringOfString:source.
+            sourcePtr := self at:(Method instVarOffsetOf:'source').
+            sourceRef := memory fetchObjectAt:sourcePtr.
+            sourceRef isString ifFalse:[
+                self halt.
             ].
+            source := memory printStringOfString:sourceRef.
             ^ source.
         ].
         ^ nil