diff --git a/.travis.yml b/.travis.yml
index ddd52549..76951843 100644
--- a/.travis.yml
+++ b/.travis.yml
@@ -1,8 +1,5 @@
 sudo: false
-cache:
-    directories:
-        - wheelhouse
-
+cache: pip
 language: python
 python:
     - 2.7
@@ -19,10 +16,8 @@ env:
     - TOXENV=pypy-pure,pypy3-pure
 
 install:
-    - pip install wheel tox
-    - ls -la wheelhouse
-    - if [ ! -f wheelhouse/Cython-0.22-cp27-none-linux_x86_64.whl ] ; then pip wheel cython==0.22 ; fi
-    - pip install wheelhouse/Cython-0.22-cp27-none-linux_x86_64.whl
+    - pip install tox
+    - pip install cython --install-option=--cython-with-refnanny --install-option=--no-cython-compile
     - cython --cplus msgpack/_packer.pyx msgpack/_unpacker.pyx
 
 script: tox
diff --git a/msgpack/_unpacker.pyx b/msgpack/_unpacker.pyx
index d53f7241..997979c1 100644
--- a/msgpack/_unpacker.pyx
+++ b/msgpack/_unpacker.pyx
@@ -37,10 +37,10 @@ cdef extern from "unpack.h":
     ctypedef struct unpack_context:
         msgpack_user user
         PyObject* obj
-        size_t count
+        Py_ssize_t count
 
     ctypedef int (*execute_fn)(unpack_context* ctx, const char* data,
-                               size_t len, size_t* off) except? -1
+                               Py_ssize_t len, Py_ssize_t* off) except? -1
     execute_fn unpack_construct
     execute_fn unpack_skip
     execute_fn read_array_header
@@ -112,7 +112,7 @@ def unpackb(object packed, object object_hook=None, object list_hook=None,
     See :class:`Unpacker` for options.
     """
     cdef unpack_context ctx
-    cdef size_t off = 0
+    cdef Py_ssize_t off = 0
     cdef int ret
 
     cdef char* buf
@@ -142,7 +142,7 @@ def unpackb(object packed, object object_hook=None, object list_hook=None,
             raise ExtraData(obj, PyBytes_FromStringAndSize(buf+off, buf_len-off))
         return obj
     else:
-        raise UnpackValueError("Unpack failed: error = %d" % (ret,))
+        raise UnpackValueError("Unpack failed: error = %s" % (ret,))
 
 
 def unpack(object stream, object object_hook=None, object list_hook=None,
@@ -233,14 +233,14 @@ cdef class Unpacker(object):
     """
     cdef unpack_context ctx
     cdef char* buf
-    cdef size_t buf_size, buf_head, buf_tail
+    cdef Py_ssize_t buf_size, buf_head, buf_tail
     cdef object file_like
     cdef object file_like_read
     cdef Py_ssize_t read_size
     # To maintain refcnt.
     cdef object object_hook, object_pairs_hook, list_hook, ext_hook
     cdef object encoding, unicode_errors
-    cdef size_t max_buffer_size
+    cdef Py_ssize_t max_buffer_size
 
     def __cinit__(self):
         self.buf = NULL
@@ -325,10 +325,10 @@ cdef class Unpacker(object):
         cdef:
             char* buf = self.buf
             char* new_buf
-            size_t head = self.buf_head
-            size_t tail = self.buf_tail
-            size_t buf_size = self.buf_size
-            size_t new_size
+            Py_ssize_t head = self.buf_head
+            Py_ssize_t tail = self.buf_tail
+            Py_ssize_t buf_size = self.buf_size
+            Py_ssize_t new_size
 
         if tail + _buf_len > buf_size:
             if ((tail - head) + _buf_len) <= buf_size:
@@ -374,7 +374,7 @@ cdef class Unpacker(object):
     cdef object _unpack(self, execute_fn execute, object write_bytes, bint iter=0):
         cdef int ret
         cdef object obj
-        cdef size_t prev_head
+        cdef Py_ssize_t prev_head
 
         if self.buf_head >= self.buf_tail and self.file_like is not None:
             self.read_from_file()
@@ -408,7 +408,7 @@ cdef class Unpacker(object):
 
     def read_bytes(self, Py_ssize_t nbytes):
         """Read a specified number of raw bytes from the stream"""
-        cdef size_t nread
+        cdef Py_ssize_t nread
         nread = min(self.buf_tail - self.buf_head, nbytes)
         ret = PyBytes_FromStringAndSize(self.buf + self.buf_head, nread)
         self.buf_head += nread
diff --git a/msgpack/unpack.h b/msgpack/unpack.h
index 5deb7cde..297bc93d 100644
--- a/msgpack/unpack.h
+++ b/msgpack/unpack.h
@@ -33,7 +33,7 @@ typedef struct unpack_user {
 typedef PyObject* msgpack_unpack_object;
 struct unpack_context;
 typedef struct unpack_context unpack_context;
-typedef int (*execute_fn)(unpack_context *ctx, const char* data, size_t len, size_t* off);
+typedef int (*execute_fn)(unpack_context *ctx, const char* data, Py_ssize_t len, Py_ssize_t* off);
 
 static inline msgpack_unpack_object unpack_callback_root(unpack_user* u)
 {
diff --git a/msgpack/unpack_template.h b/msgpack/unpack_template.h
index d34eceda..5b389b81 100644
--- a/msgpack/unpack_template.h
+++ b/msgpack/unpack_template.h
@@ -24,8 +24,8 @@
 
 typedef struct unpack_stack {
     PyObject* obj;
-    size_t size;
-    size_t count;
+    Py_ssize_t size;
+    Py_ssize_t count;
     unsigned int ct;
     PyObject* map_key;
 } unpack_stack;
@@ -72,7 +72,7 @@ static inline PyObject* unpack_data(unpack_context* ctx)
 
 
 template <bool construct>
-static inline int unpack_execute(unpack_context* ctx, const char* data, size_t len, size_t* off)
+static inline int unpack_execute(unpack_context* ctx, const char* data, Py_ssize_t len, Py_ssize_t* off)
 {
     assert(len >= *off);
 
@@ -89,7 +89,7 @@ static inline int unpack_execute(unpack_context* ctx, const char* data, size_t l
     */
     unpack_user* user = &ctx->user;
 
-    PyObject* obj;
+    PyObject* obj = NULL;
     unpack_stack* c = NULL;
 
     int ret;
@@ -409,7 +409,7 @@ static inline int unpack_execute(unpack_context* ctx, const char* data, size_t l
 #undef start_container
 
 template <unsigned int fixed_offset, unsigned int var_offset>
-static inline int unpack_container_header(unpack_context* ctx, const char* data, size_t len, size_t* off)
+static inline int unpack_container_header(unpack_context* ctx, const char* data, Py_ssize_t len, Py_ssize_t* off)
 {
     assert(len >= *off);
     uint32_t size;