--- a/Tools__NewSystemBrowser.st	Wed Feb 14 17:28:13 2018 +0100
+++ b/Tools__NewSystemBrowser.st	Wed Feb 14 17:32:42 2018 +0100
@@ -36643,13 +36643,16 @@
 
     currentClass := self theSingleSelectedLoadedNonMetaclassOrNil.
     currentClass isNil ifTrue:[
-        self warn:'Cannot compare unloaded classes.'.
-        ^ self.
+        currentClass := self theSingleSelectedClass.
+        currentClass isNil ifTrue:[
+            self warn:'Please select a single class.'.
+            ^ self.
+        ].
     ].
 
     nm := currentClass name.
-    mgr := manager.
-    mgr isNil ifTrue:[
+    (mgr := manager) isNil ifTrue:[
+        self warn:'No source code manager for: ',nm.
         ^ self
     ].