py3/keksschema.py | 2 ++

diff --git a/py3/keksschema.py b/py3/keksschema.py
index b96d33f9ed9d654bdffd051d0c34a5a326a3c6a0620cf523352f9d47d2bcd2bb..3824954faa0fce6373b59cb53d216f28fd72b06330cab9827afc2037c99a2cc4 100755
--- a/py3/keksschema.py
+++ b/py3/keksschema.py
@@ -95,6 +95,8 @@                     raise ValidationError("%s: %d: P: %r %r" % (sname, cmdIdx, arg, v))
         elif cmd == "x": # EXAMINED
             needExaminedCheck = True
             if len(vs) != 0:
+                if nonExamined == 0:
+                    raise ValueError("%s: %d: x: already zero nonExamined" % (sname, cmdIdx))
                 nonExamined -= 1
         else:
             raise ValueError("%s: %d: unknown cmd: %r" % (sname, cmdIdx, cmd))