diff --git a/cytoolz/itertoolz.pyx b/cytoolz/itertoolz.pyx
index e138f71..6c5bc88 100644
--- a/cytoolz/itertoolz.pyx
+++ b/cytoolz/itertoolz.pyx
@@ -960,12 +960,13 @@ cdef class sliding_window:
     [1.5, 2.5, 3.5]
     """
     def __cinit__(self, Py_ssize_t n, object seq):
         cdef Py_ssize_t i
         self.iterseq = iter(seq)
         self.prev = PyTuple_New(n)
+        PyTuple_SET_ITEM(self.prev, 0, None)
         for i, seq in enumerate(islice(self.iterseq, n-1), 1):
             Py_INCREF(seq)
             PyTuple_SET_ITEM(self.prev, i, seq)
         self.n = n
 
     def __iter__(self):
@@ -1051,12 +1052,14 @@ cdef class partition_all:
             i += 1
             if i == self.n:
                 return result
         # iterable exhausted before filling the tuple
         if i == 0:
             raise StopIteration
+        for j in range(i, self.n):
+            PyTuple_SET_ITEM(result, j, None)
         return PyTuple_GetSlice(result, 0, i)
 
 
 cpdef object count(object seq):
     """
     Count the number of items in seq