--- a/hgext/mq.py	Sun May 18 23:31:54 2008 +0200
+++ b/hgext/mq.py	Sun May 18 23:49:33 2008 +0200
@@ -126,6 +126,8 @@
                 self.series_guards.append(self.guard_re.findall(comment))
 
     def check_guard(self, guard):
+        if not guard:
+            return _('guard cannot be an empty string')
         bad_chars = '# \t\r\n\f'
         first = guard[0]
         for c in '-+':

--- a/tests/test-mq-guards	Sun May 18 23:31:54 2008 +0200
+++ b/tests/test-mq-guards	Sun May 18 23:49:33 2008 +0200
@@ -44,6 +44,8 @@
 hg qpush
 
 hg qpop
+echo % test selection of an empty guard
+hg qselect ""
 hg qselect a
 echo % should push a.patch
 hg qpush

--- a/tests/test-mq-guards.out	Sun May 18 23:31:54 2008 +0200
+++ b/tests/test-mq-guards.out	Sun May 18 23:49:33 2008 +0200
@@ -15,6 +15,8 @@
 applying b.patch
 Now at: b.patch
 Patch queue now empty
+% test selection of an empty guard
+abort: guard cannot be an empty string
 number of unguarded, unapplied patches has changed from 2 to 3
 % should push a.patch
 applying a.patch

--- a/tests/test-mq.out	Sun May 18 23:31:54 2008 +0200
+++ b/tests/test-mq.out	Sun May 18 23:49:33 2008 +0200
@@ -28,7 +28,7 @@
  qclone       clone main and patch repository at same time
  qcommit      commit changes in the queue repository
  qdelete      remove patches from queue
- qdiff        diff of the current patch
+ qdiff        diff of the current patch and subsequent modifications
  qfold        fold the named patches into the current patch
  qgoto        push or pop patches until named patch is at top of stack
  qguard       set or print guards for a patch