diff -r 96b8395ca393 nml/free_number_list.py
--- a/nml/free_number_list.py	Thu May 18 20:58:11 2017 +0200
+++ b/nml/free_number_list.py	Mon Jun 19 20:26:10 2017 +0200
@@ -69,7 +69,7 @@
         @return: Some free number.
         """
         assert len(self.states) > 0
-        if self.free_numbers == 0:
+        if len(self.free_numbers) == 0:
             raise generic.ScriptError(self.exception, pos)
         num = self.free_numbers.pop()
         self.states[-1].append(num)