--- a/ProjectBuilder.st	Wed Feb 04 19:07:49 2015 +0100
+++ b/ProjectBuilder.st	Sat Feb 07 17:00:05 2015 +0100
@@ -849,7 +849,9 @@
             |cls fileName newSource|
 
             cls := Smalltalk classNamed:eachClassName.
-            self assert:cls isLoaded.
+            (cls notNil and:[cls isLoaded]) ifFalse:[
+                self error:'missing class: ',eachClassName mayProceed:true
+            ].
             fileName := (Smalltalk fileNameForClass:cls),'.st'.
             fileName := packageTargetDir asFilename construct:fileName.
             fileName exists ifTrue:[