diff --git a/jupyterhub/app.py b/jupyterhub/app.py
index 5544a8de..ca0e6f3f 100644
--- a/jupyterhub/app.py
+++ b/jupyterhub/app.py
@@ -50,7 +50,7 @@ from .handlers.static import CacheControlStaticFilesHandler
 from . import orm
 from ._data import DATA_FILES_PATH
 from .log import CoroutineLogFormatter, log_request
-from .traitlets import URLPrefix
+from .traitlets import URLPrefix, Command
 from .utils import (
     url_path_join,
     ISO8601_ms, ISO8601_s,
@@ -237,7 +237,7 @@ class JupyterHub(Application):
         help="Supply extra arguments that will be passed to Jinja environment."
     )
     
-    proxy_cmd = Unicode('configurable-http-proxy', config=True,
+    proxy_cmd = Command('configurable-http-proxy', config=True,
         help="""The command to start the http proxy.
         
         Only override if configurable-http-proxy is not on your PATH
@@ -742,7 +742,7 @@ class JupyterHub(Application):
 
         env = os.environ.copy()
         env['CONFIGPROXY_AUTH_TOKEN'] = self.proxy.auth_token
-        cmd = [self.proxy_cmd,
+        cmd = self.proxy_cmd + [
             '--ip', self.proxy.public_server.ip,
             '--port', str(self.proxy.public_server.port),
             '--api-ip', self.proxy.api_server.ip,
diff --git a/jupyterhub/spawner.py b/jupyterhub/spawner.py
index 46be3a2a..bdf4f351 100644
--- a/jupyterhub/spawner.py
+++ b/jupyterhub/spawner.py
@@ -21,6 +21,7 @@ from IPython.utils.traitlets import (
     Any, Bool, Dict, Enum, Instance, Integer, Float, List, Unicode,
 )
 
+from .traitlets import Command
 from .utils import random_port
 
 NUM_PAT = re.compile(r'\d+')
@@ -93,7 +94,7 @@ class Spawner(LoggingConfigurable):
         env['JPY_API_TOKEN'] = self.api_token
         return env
     
-    cmd = List(Unicode, default_value=['jupyterhub-singleuser'], config=True,
+    cmd = Command(['jupyterhub-singleuser'], config=True,
         help="""The command used for starting notebooks."""
     )
     args = List(Unicode, config=True,
diff --git a/jupyterhub/tests/test_traitlets.py b/jupyterhub/tests/test_traitlets.py
new file mode 100644
index 00000000..5c1fa895
--- /dev/null
+++ b/jupyterhub/tests/test_traitlets.py
@@ -0,0 +1,26 @@
+from traitlets import HasTraits
+from jupyterhub.traitlets import URLPrefix, Command
+
+def test_url_prefix():
+    class C(HasTraits):
+        url = URLPrefix()
+    
+    c = C()
+    c.url = '/a/b/c/'
+    assert c.url == '/a/b/c/'
+    c.url = '/a/b'
+    assert c.url == '/a/b/'
+    c.url = 'a/b/c/d'
+    assert c.url == '/a/b/c/d/'
+
+def test_command():
+    class C(HasTraits):
+        cmd = Command('default command')
+        cmd2 = Command(['default_cmd'])
+    
+    c = C()
+    assert c.cmd == ['default command']
+    assert c.cmd2 == ['default_cmd']
+    c.cmd = 'foo bar'
+    assert c.cmd == ['foo bar']
+
diff --git a/jupyterhub/traitlets.py b/jupyterhub/traitlets.py
index 065ac1d5..d838895c 100644
--- a/jupyterhub/traitlets.py
+++ b/jupyterhub/traitlets.py
@@ -2,7 +2,7 @@
 # Copyright (c) Jupyter Development Team.
 # Distributed under the terms of the Modified BSD License.
 
-from IPython.utils.traitlets import Unicode
+from IPython.utils.traitlets import List, Unicode
 
 class URLPrefix(Unicode):
     def validate(self, obj, value):
@@ -12,3 +12,18 @@ class URLPrefix(Unicode):
         if not u.endswith('/'):
             u = u + '/'
         return u
+
+class Command(List):
+    """Traitlet for a command that should be a list of strings,
+    but allows it to be specified as a single string.
+    """
+    def __init__(self, default_value=None, **kwargs):
+        kwargs.setdefault('minlen', 1)
+        if isinstance(default_value, str):
+            default_value = [default_value]
+        super().__init__(Unicode, default_value, **kwargs)
+        
+    def validate(self, obj, value):
+        if isinstance(value, str):
+            value = [value]
+        return super().validate(obj, value)