diff --git a/dumbo/core.py b/dumbo/core.py
index 56c28e0..1c12ac8 100644
--- a/dumbo/core.py
+++ b/dumbo/core.py
@@ -463,7 +463,7 @@ def valwrapper(data, valfunc):
 
 def mapfunc_iter(data, mapfunc):
     for (key, value) in data:
-        for output in mapfunc(key, value):
+        for output in mapfunc(key, value) or ():
             yield output
 
 
@@ -478,7 +478,7 @@ def itermap(data, mapfunc, valfunc=None):
 
 def redfunc_iter(data, redfunc):
     for (key, values) in data:
-        for output in redfunc(key, values):
+        for output in redfunc(key, values) or ():
             yield output
 
 
diff --git a/dumbo/lib/__init__.py b/dumbo/lib/__init__.py
index 2193388..fc066e6 100644
--- a/dumbo/lib/__init__.py
+++ b/dumbo/lib/__init__.py
@@ -17,6 +17,7 @@
 import heapq
 import os
 import types
+import inspect
 from itertools import chain, imap, izip
 from math import sqrt
 from copy import copy
@@ -128,7 +129,7 @@ def __call__normalkey(self, data):
             path, key = key
             for pattern, mapper in mappers:
                 if pattern in path:
-                    for output in mapper(key, value):
+                    for output in mapper(key, value, path=path):
                         yield output
 
     def __call__joinkey(self, data):
@@ -138,7 +139,7 @@ def __call__joinkey(self, data):
             key.body = key.body[1]
             for pattern, mapper in mappers:
                 if pattern in path:
-                    for output in mapper(key, value):
+                    for output in mapper(key, value, path=path):
                         yield output
 
     def add(self, pattern, mapper):
@@ -169,15 +170,20 @@ def configure(self):
             mapper.configure()
         if hasattr(mapper, 'close'):
             self.closefunc = mapper.close
+        mapper_call = mapper
+        if isinstance(mapper_call, mrbase_class):
+            mapper_call = mapper.__call__
         self.mapper = mapper
+        if not inspect.getargspec(mapper_call).keywords:
+            self.mapper = lambda key, value, **kwargs: mapper(key, value)
 
     def close(self):
         if self.closefunc:
             self.closefunc()
 
-    def __call__(self, key, value):
+    def __call__(self, key, value, **kwargs):
         key.isprimary = self.isprimary
-        for k, v in self.mapper(key.body, value):
+        for k, v in self.mapper(key.body, value, **kwargs) or ():
             jk = copy(key)
             jk.body = k
             yield jk, v
@@ -202,14 +208,12 @@ class JoinCombiner(object):
     def __call__(self, key, values):
         if key.isprimary:
             self._key = key.body
-            output = self.primary(key.body, values)
-            if output:
-                for k, v in output:
-                    jk = copy(key)
-                    jk.body = k
-                    yield jk, v
+            for k, v in self.primary(key.body, values) or ():
+                jk = copy(key)
+                jk.body = k
+                yield jk, v
         elif not self.secondary_blocked(key.body):
-            for k, v in self.secondary(key.body, values):
+            for k, v in self.secondary(key.body, values) or ():
                 jk = copy(key)
                 jk.body = k
                 yield jk, v