diff --git a/mercurial/dispatch.py b/mercurial/dispatch.py
--- a/mercurial/dispatch.py
+++ b/mercurial/dispatch.py
@@ -248,7 +248,7 @@ class cmdalias(object):
         if self.shadows:
             ui.debug("alias '%s' shadows command\n" % self.name)
 
-        return self.fn(ui, *args, **opts)
+        return util.checksignature(self.fn)(ui, *args, **opts)
 
 def addaliases(ui, cmdtable):
     # aliases are processed after extensions have been loaded, so they
diff --git a/tests/test-alias b/tests/test-alias
--- a/tests/test-alias
+++ b/tests/test-alias
@@ -14,6 +14,7 @@ shortlog = log --template '{rev} {node|s
 dln = lognull --debug
 nousage = rollback
 put = export -r 0 -o "\$FOO/%R.diff"
+rt = root
 
 [defaults]
 mylog = -q
@@ -64,3 +65,8 @@ hg dln
 echo '% path expanding'
 FOO=`pwd` hg put
 cat 0.diff
+
+echo '% invalid arguments'
+hg rt foo
+
+exit 0
diff --git a/tests/test-alias.out b/tests/test-alias.out
--- a/tests/test-alias.out
+++ b/tests/test-alias.out
@@ -43,3 +43,16 @@ diff -r 000000000000 -r e63c23eaa88a foo
 +++ b/foo	Thu Jan 01 00:00:00 1970 +0000
 @@ -0,0 +1,1 @@
 +foo
+% invalid arguments
+hg rt: invalid arguments
+hg rt 
+
+alias for: hg root
+
+print the root (top) of the current working directory
+
+    Print the root directory of the current repository.
+
+    Returns 0 on success.
+
+use "hg -v help rt" to show global options