diff --git a/IPython/core/shellapp.py b/IPython/core/shellapp.py
index 50dcad3..032dd97 100644
--- a/IPython/core/shellapp.py
+++ b/IPython/core/shellapp.py
@@ -201,7 +201,7 @@ class InteractiveShellApp(Configurable):
     )
     shell = Instance('IPython.core.interactiveshell.InteractiveShellABC')
     
-    user_ns = Dict(default_value=None)
+    user_ns = Instance(dict, args=None, allow_none=True)
     def _user_ns_changed(self, name, old, new):
         if self.shell is not None:
             self.shell.user_ns = new
diff --git a/IPython/kernel/zmq/ipkernel.py b/IPython/kernel/zmq/ipkernel.py
index a65b05d..ec0d8be 100755
--- a/IPython/kernel/zmq/ipkernel.py
+++ b/IPython/kernel/zmq/ipkernel.py
@@ -87,7 +87,7 @@ class Kernel(Configurable):
         if self.shell is not None:
             self.shell.user_module = new
     
-    user_ns = Dict(default_value=None)
+    user_ns = Instance(dict, args=None, allow_none=True)
     def _user_ns_changed(self, name, old, new):
         if self.shell is not None:
             self.shell.user_ns = new