diff --git a/hgext/schemes.py b/hgext/schemes.py
--- a/hgext/schemes.py
+++ b/hgext/schemes.py
@@ -72,9 +72,10 @@ class ShortRepository(object):
         return hg._peerlookup(url).instance(ui, url, create)
 
 def hasdriveletter(orig, path):
-    for scheme in schemes:
-        if path.startswith(scheme + ':'):
-            return False
+    if path:
+        for scheme in schemes:
+            if path.startswith(scheme + ':'):
+                return False
     return orig(path)
 
 schemes = {
diff --git a/mercurial/util.py b/mercurial/util.py
--- a/mercurial/util.py
+++ b/mercurial/util.py
@@ -1629,6 +1629,8 @@ class url(object):
         'path'
         >>> str(url('file:///tmp/foo/bar'))
         'file:///tmp/foo/bar'
+        >>> str(url('file:///c:/tmp/foo/bar'))
+        'file:///c%3A/tmp/foo/bar'
         >>> print url(r'bundle:foo\bar')
         bundle:foo\bar
         """
@@ -1643,8 +1645,11 @@ class url(object):
         s = self.scheme + ':'
         if self.user or self.passwd or self.host:
             s += '//'
-        elif self.scheme and (not self.path or self.path.startswith('/')):
+        elif self.scheme and (not self.path or self.path.startswith('/')
+                              or hasdriveletter(self.path)):
             s += '//'
+            if hasdriveletter(self.path):
+                s += '/'
         if self.user:
             s += urllib.quote(self.user, safe=self._safechars)
         if self.passwd:
@@ -1716,7 +1721,7 @@ def hasscheme(path):
     return bool(url(path).scheme)
 
 def hasdriveletter(path):
-    return path[1:2] == ':' and path[0:1].isalpha()
+    return path and path[1:2] == ':' and path[0:1].isalpha()
 
 def urllocalpath(path):
     return url(path, parsequery=False, parsefragment=False).localpath()
diff --git a/tests/test-url.py b/tests/test-url.py
--- a/tests/test-url.py
+++ b/tests/test-url.py
@@ -219,7 +219,7 @@ def test_url():
     >>> u
     <url scheme: 'file', path: 'f:oo/bar/baz'>
     >>> str(u)
-    'file:f%3Aoo/bar/baz'
+    'file:///f%3Aoo/bar/baz'
     >>> u.localpath()
     'f:oo/bar/baz'