diff --git a/requests/exceptions.py b/requests/exceptions.py
index f45d9c76..2f093baf 100644
--- a/requests/exceptions.py
+++ b/requests/exceptions.py
@@ -104,12 +104,15 @@ class StreamConsumedError(RequestException, TypeError):
 class RetryError(RequestException):
     """Custom retries logic failed"""
 
+
 class UnrewindableBodyError(RequestException):
     """Requests encountered an error when trying to rewind a body"""
 
+
 class ConflictingHeaderError(RequestException):
     """Mutually exclusive request headers set"""
 
+
 class InvalidBodyError(RequestException, ValueError):
     """An invalid request body was specified"""
 
diff --git a/requests/models.py b/requests/models.py
index c1b8ba0c..2b3e1fe3 100644
--- a/requests/models.py
+++ b/requests/models.py
@@ -39,7 +39,7 @@ from .utils import (
     guess_filename, get_auth_from_url, requote_uri,
     stream_decode_response_unicode, to_key_val_list, parse_header_links,
     iter_slices, guess_json_utf, super_len, check_header_validity,
-    determine_if_stream)
+    is_stream)
 from .compat import (
     cookielib, urlunparse, urlsplit, urlencode, str, bytes, StringIO,
     is_py2, chardet, builtin_str, basestring)
@@ -468,9 +468,7 @@ class PreparedRequest(RequestEncodingMixin, RequestHooksMixin):
             if not isinstance(body, bytes):
                 body = body.encode('utf-8')
 
-        is_stream = determine_if_stream(data)
-
-        if is_stream:
+        if is_stream(data):
             body = data
 
             if getattr(body, 'tell', None) is not None:
@@ -509,30 +507,26 @@ class PreparedRequest(RequestEncodingMixin, RequestHooksMixin):
     def prepare_content_length(self, body):
         """Prepares Content-Length header.
         
-        If the length of the body of the request can be computed, Content-Length is set using
-        super_len. If user has manually set either a Transfer-Encoding or Content-Length header
-        when it should not be set (they should be mutually exclusive) an ConflictingHeaderError
+        If the length of the body of the request can be computed, Content-Length
+        is set using ``super_len``. If user has manually set either a
+        Transfer-Encoding or Content-Length header when it should not be set
+        (they should be mutually exclusive) a ConflictingHeaderError
         error will be raised.
         """
         if body is not None:
-            is_stream = determine_if_stream(body)
-
-            try:
-                length = super_len(body)
-            except (TypeError, AttributeError, UnsupportedOperation):
-                length = None
+            length = super_len(body)
 
             if length:
                 self.headers['Content-Length'] = builtin_str(length)
-            elif is_stream and not length:
+            elif is_stream(body):
                 self.headers['Transfer-Encoding'] = 'chunked'
             else:
-                raise InvalidBodyError("Non-null body must have length or be streamable")
-        elif (self.method not in ('GET', 'HEAD')) and (self.headers.get('Content-Length') is None):
+                raise InvalidBodyError('Non-null body must have length or be streamable.')
+        elif self.method not in ('GET', 'HEAD') and self.headers.get('Content-Length') is None:
             self.headers['Content-Length'] = '0'
 
         if 'Transfer-Encoding' in self.headers and 'Content-Length' in self.headers:
-            raise ConflictingHeaderError('Transfer-Encoding and Content-Length headers both set')
+            raise ConflictingHeaderError('Transfer-Encoding and Content-Length headers both set.')
 
     def prepare_auth(self, auth, url=''):
         """Prepares the given HTTP auth data."""
diff --git a/requests/utils.py b/requests/utils.py
index e541a626..fff14db7 100644
--- a/requests/utils.py
+++ b/requests/utils.py
@@ -881,9 +881,8 @@ def rewind_body(prepared_request):
         raise UnrewindableBodyError("Unable to rewind request body for redirect.")
 
 
-def determine_if_stream(data):
-    """Given data, determines if it should be sent as a stream.
-    """
+def is_stream(data):
+    """Given data, determines if it should be sent as a stream."""
     is_iterable = getattr(data, '__iter__', False)
-    is_io_type = not isinstance(data, (basestring, list, tuple, dict))
+    is_io_type = not isinstance(data, (basestring, list, tuple, collections.Mapping))
     return is_iterable and is_io_type
diff --git a/tests/test_requests.py b/tests/test_requests.py
index a9de9c17..3beb861d 100755
--- a/tests/test_requests.py
+++ b/tests/test_requests.py
@@ -1925,16 +1925,18 @@ class TestRequests:
         assert 'Content-Length' not in prepared_request.headers
 
     def test_chunked_upload_with_manually_set_content_length_header_raises_error(self, httpbin):
-        """Ensure that if a user manually sets a content length header when the data
-        is chunked that an ConflictingHeaderError is raised"""
-        data = (i for i in [b'a', b'b', b'c'])
+        """Ensure that if a user manually sets a content length header, when
+        the data is chunked, that a ConflictingHeaderError is raised.
+        """
+        data = (i for i in [b'a', b'b', b'c']) 
         url = httpbin('post')
         with pytest.raises(ConflictingHeaderError):
             r = requests.post(url, data=data, headers={'Content-Length': 'foo'})
 
     def test_content_length_with_manually_set_transfer_encoding_raises_error(self, httpbin):
-        """Ensure that if a user manually sets a Transfer-Encoding header when data is not chunked
-        that an ConflictingHeaderError is raised"""
+        """Ensure that if a user manually sets a Transfer-Encoding header when
+        data is not chunked that an ConflictingHeaderError is raised.
+        """
         data = 'test data'
         url = httpbin('post')
         with pytest.raises(ConflictingHeaderError):
@@ -1945,7 +1947,7 @@ class TestRequests:
         try:
             requests.post(url, data=None)
         except ConflictingHeaderError:
-            pytest.fail('ConflictingHeaderError raised')
+            pytest.fail('ConflictingHeaderError raised.')
 
     def test_custom_redirect_mixin(self, httpbin):
         """Tests a custom mixin to overwrite ``get_redirect_target``.