diff --git a/requests/models.py b/requests/models.py
index 3f6a4f92..2370b67f 100644
--- a/requests/models.py
+++ b/requests/models.py
@@ -472,7 +472,7 @@ class PreparedRequest(RequestEncodingMixin, RequestHooksMixin):
             l = super_len(body)
             if l:
                 self.headers['Content-Length'] = builtin_str(l)
-        elif self.method not in ('GET', 'HEAD'):
+        elif (self.method not in ('GET', 'HEAD')) and (self.headers.get('Content-Length') is None):
             self.headers['Content-Length'] = '0'
 
     def prepare_auth(self, auth, url=''):