diff --git a/requests/exceptions.py b/requests/exceptions.py
index cddffd95..f2ac3a16 100644
--- a/requests/exceptions.py
+++ b/requests/exceptions.py
@@ -110,6 +110,8 @@ class UnrewindableBodyError(RequestException):
 class ConflictingHeaderError(RequestException):
     """Mutually exclusive request headers set"""
 
+class UnreachableCodeError(RequestException, RuntimeError):
+    """Unreachable code block reached"""
 
 # Warnings
 
diff --git a/requests/models.py b/requests/models.py
index eb68289b..60e6ca94 100644
--- a/requests/models.py
+++ b/requests/models.py
@@ -527,7 +527,7 @@ class PreparedRequest(RequestEncodingMixin, RequestHooksMixin):
             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"
+                raise UnreachableCodeError("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'
 
diff --git a/requests/utils.py b/requests/utils.py
index e36d62af..e541a626 100644
--- a/requests/utils.py
+++ b/requests/utils.py
@@ -884,6 +884,6 @@ def rewind_body(prepared_request):
 def determine_if_stream(data):
     """Given data, determines if it should be sent as a stream.
     """
-    is_iterable = hasattr(data, '__iter__')
+    is_iterable = getattr(data, '__iter__', False)
     is_io_type = not isinstance(data, (basestring, list, tuple, dict))
     return is_iterable and is_io_type