--- a/ClassBuilder.st	Thu Dec 17 20:05:27 2015 +0100
+++ b/ClassBuilder.st	Fri Dec 18 13:48:26 2015 +0100
@@ -2101,10 +2101,13 @@
 
     (className isSymbol not
     or:[className size == 0]) ifTrue:[
-        ClassBuildError raiseErrorString:'invalid class name (must be a nonEmpty symbol)'.
+        ClassBuildError raiseErrorString:'Invalid class name (must be a nonEmpty symbol)'.
     ].
     (className first isLetterOrUnderline) ifFalse:[
-        ClassBuildError raiseErrorString:'invalid class name (must start with a letter)'.
+        ClassBuildError raiseErrorString:'Invalid class name (must start with a letter)'.
+    ].
+    (className conform:[:ch | ch isLetterOrDigit or:[ch == $_]]) ifFalse:[
+        ClassBuildError raiseErrorString:'Invalid class name (must consist of letters, digits or underline)'.
     ].
 
     "Modified: / 11-08-2006 / 12:46:54 / cg"