diff --git a/jsonschema.py b/jsonschema.py
index 6e8551d..10e9adf 100644
--- a/jsonschema.py
+++ b/jsonschema.py
@@ -525,18 +525,25 @@ class RefResolver(object):
     """
     Resolve JSON References.
 
+    :argument string base_uri: URI of the referring document
+    :argument referrer: the actual referring document
+    :argument dict store: a mapping from URIs to documents to cache
+
     """
 
     def __init__(self, base_uri, referrer, store=()):
         self.base_uri = base_uri
         self.referrer = referrer
-        self.store = collections.defaultdict(dict, store, **_meta_schemas())
+        self.store = dict(store, **_meta_schemas())
 
     @classmethod
     def from_schema(cls, schema, *args, **kwargs):
         """
         Construct a resolver from a JSON schema object.
 
+        :argument schema schema: the referring schema
+        :rtype: :class:`RefResolver`
+
         """
 
         return cls(schema.get("id", ""), schema, *args, **kwargs)
@@ -545,6 +552,9 @@ class RefResolver(object):
         """
         Resolve a JSON ``ref``.
 
+        :argument string ref: reference to resolve
+        :returns: the referrant document
+
         """
 
         base_uri = self.base_uri
@@ -563,6 +573,9 @@ class RefResolver(object):
         """
         Resolve a ``fragment`` within the referenced ``document``.
 
+        :argument document: the referrant document
+        :argument string fragment: a URI fragment to resolve within it
+
         """
 
         parts = unquote(fragment).split("/") if fragment else []
@@ -583,6 +596,9 @@ class RefResolver(object):
 
         Does not check the store first.
 
+        :argument string uri: the URI to resolve
+        :returns: the retrieved document
+
         """
 
         return json.load(urlopen(uri))