diff --git a/requests/models.py b/requests/models.py
index 365c0ed8..eb68289b 100644
--- a/requests/models.py
+++ b/requests/models.py
@@ -526,6 +526,8 @@ class PreparedRequest(RequestEncodingMixin, RequestHooksMixin):
                 self.headers['Content-Length'] = builtin_str(length)
             elif is_stream and not length:
                 self.headers['Transfer-Encoding'] = 'chunked'
+            else:
+                assert False, "If body is not null, it must either have a length or be streamable"
         elif (self.method not in ('GET', 'HEAD')) and (self.headers.get('Content-Length') is None):
             self.headers['Content-Length'] = '0'