diff --git a/hgext/commitextras.py b/hgext/commitextras.py
--- a/hgext/commitextras.py
+++ b/hgext/commitextras.py
@@ -9,6 +9,8 @@
 
 from __future__ import absolute_import
 
+import re
+
 from mercurial.i18n import _
 from mercurial import (
     commands,
@@ -52,6 +54,10 @@ def _commit(orig, ui, repo, *pats, **opt
                                 "KEY=VALUE format")
                         raise error.Abort(msg % raw)
                     k, v = raw.split('=', 1)
+                    if re.search('[^\w-]', k):
+                        msg = _("keys can only contain ascii letters, digits,"
+                                " '_' and '-'")
+                        raise error.Abort(msg)
                     if k in usedinternally:
                         msg = _("key '%s' is used internally, can't be set "
                                 "manually")
diff --git a/tests/test-commit.t b/tests/test-commit.t
--- a/tests/test-commit.t
+++ b/tests/test-commit.t
@@ -135,6 +135,9 @@ Using the advanced --extra flag
   $ hg commit -m "adding internal used extras" --extra amend_source=hash
   abort: key 'amend_source' is used internally, can't be set manually
   [255]
+  $ hg commit -m "special chars in extra" --extra id@phab=214
+  abort: keys can only contain ascii letters, digits, '_' and '-'
+  [255]
   $ hg commit -m "adding extras" --extra sourcehash=foo --extra oldhash=bar
   $ hg log -r . -T '{extras % "{extra}\n"}'
   branch=default