diff --git a/requests/exceptions.py b/requests/exceptions.py
index 2f093baf..314783b7 100644
--- a/requests/exceptions.py
+++ b/requests/exceptions.py
@@ -109,10 +109,6 @@ 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 2b3e1fe3..ac348e9d 100644
--- a/requests/models.py
+++ b/requests/models.py
@@ -33,7 +33,7 @@ from .packages.urllib3.exceptions import (
 from .exceptions import (
     HTTPError, MissingScheme, InvalidURL, ChunkedEncodingError,
     ContentDecodingError, ConnectionError, StreamConsumedError,
-    ConflictingHeaderError)
+    InvalidHeader, InvalidBodyError)
 from ._internal_utils import to_native_string, unicode_is_ascii
 from .utils import (
     guess_filename, get_auth_from_url, requote_uri,
@@ -510,7 +510,7 @@ class PreparedRequest(RequestEncodingMixin, RequestHooksMixin):
         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
+        (they should be mutually exclusive) an InvalidHeader
         error will be raised.
         """
         if body is not None:
@@ -523,10 +523,13 @@ class PreparedRequest(RequestEncodingMixin, RequestHooksMixin):
             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:
+            # Set Content-Length to 0 for methods that can have a body
+            # but don't provide one. (i.e. not GET or HEAD)
             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 InvalidHeader('Conflicting Headers: Both Transfer-Encoding and '
+                                'Content-Length are set.')
 
     def prepare_auth(self, auth, url=''):
         """Prepares the given HTTP auth data."""
diff --git a/tests/test_requests.py b/tests/test_requests.py
index 3beb861d..780ebbbc 100755
--- a/tests/test_requests.py
+++ b/tests/test_requests.py
@@ -25,7 +25,7 @@ from requests.cookies import (
 from requests.exceptions import (
     ConnectionError, ConnectTimeout, InvalidScheme, InvalidURL,
     MissingScheme, ReadTimeout, Timeout, RetryError, TooManyRedirects,
-    ProxyError, InvalidHeader, UnrewindableBodyError, ConflictingHeaderError)
+    ProxyError, InvalidHeader, UnrewindableBodyError)
 from requests.models import PreparedRequest
 from requests.structures import CaseInsensitiveDict
 from requests.sessions import SessionRedirectMixin
@@ -1926,28 +1926,28 @@ class TestRequests:
 
     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 a ConflictingHeaderError is raised.
+        the data is chunked, that an InvalidHeader error is raised.
         """
         data = (i for i in [b'a', b'b', b'c']) 
         url = httpbin('post')
-        with pytest.raises(ConflictingHeaderError):
+        with pytest.raises(InvalidHeader):
             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.
+        data is not chunked that an InvalidHeader error is raised.
         """
         data = 'test data'
         url = httpbin('post')
-        with pytest.raises(ConflictingHeaderError):
+        with pytest.raises(InvalidHeader):
             r = requests.post(url, data=data, headers={'Transfer-Encoding': 'chunked'})
 
     def test_null_body_does_not_raise_error(self, httpbin):
         url = httpbin('post')
         try:
             requests.post(url, data=None)
-        except ConflictingHeaderError:
-            pytest.fail('ConflictingHeaderError raised.')
+        except InvalidHeader:
+            pytest.fail('InvalidHeader error raised unexpectedly.')
 
     def test_custom_redirect_mixin(self, httpbin):
         """Tests a custom mixin to overwrite ``get_redirect_target``.