src/Pure/library.scala

--- a/src/Pure/library.scala	Thu May 04 14:58:19 2017 +0200
+++ b/src/Pure/library.scala	Thu May 04 15:10:51 2017 +0200
@@ -145,7 +145,7 @@
   def trim_substring(s: String): String = new String(s.toCharArray)
 
   def proper_string(s: String): Option[String] =
-    if (s == "") None else Some(s)
+    if (s == null || s == "") None else Some(s)
 
 
   /* quote */