diff --git a/IPython/nbformat/current.py b/IPython/nbformat/current.py
index 16a4938..0df9f23 100644
--- a/IPython/nbformat/current.py
+++ b/IPython/nbformat/current.py
@@ -62,7 +62,7 @@ def parse_py(s, **kwargs):
 
 def reads_json(s, **kwargs):
     """Read a JSON notebook from a string and return the NotebookNode object."""
-    return convert(reader.reads(nb), current_nbformat)
+    return convert(reader.reads(s), current_nbformat)
 
 
 def writes_json(nb, **kwargs):
diff --git a/IPython/nbformat/reader.py b/IPython/nbformat/reader.py
index 23d15ed..7993cd5 100644
--- a/IPython/nbformat/reader.py
+++ b/IPython/nbformat/reader.py
@@ -18,9 +18,15 @@ Authors:
 
 import json
 
-versions = {}
-for i in range(3):
-    versions[i+1] = __import__('v{0}'.format(i+1))
+import v1
+import v2
+import v3
+
+versions = {
+    1: v1,
+    2: v2,
+    3: v3,
+    }
 
 #-----------------------------------------------------------------------------
 # Code
@@ -76,7 +82,7 @@ def reads(s, format='ipynb', **kwargs):
     nb_dict = parse_json(s, **kwargs)
     (major, minor) = get_version(nb_dict)
     if major in versions:
-        return versions[major].to_notebook_json(nb, minor=minor)
+        return versions[major].to_notebook_json(nb_dict, minor=minor)
     else:
         raise NBFormatError('Unsupported nbformat version %s' % major)
 
diff --git a/IPython/nbformat/v1/__init__.py b/IPython/nbformat/v1/__init__.py
index 3f13769..b3a51a6 100644
--- a/IPython/nbformat/v1/__init__.py
+++ b/IPython/nbformat/v1/__init__.py
@@ -20,4 +20,4 @@ from .nbjson import reads as reads_json, writes as writes_json
 from .nbjson import reads as read_json, writes as write_json
 from .nbjson import to_notebook as to_notebook_json
 
-from .convert import downgrade, upgrade
+from .convert import upgrade