|   | 1 |  initial version  | 
For dealing with differential forms with Sage, I would recommend to use the manifold implementation instead of the CoordinatePatch version, which is being deprecated (cf. the Trac ticket #24444). Could you please try with this version and tell whether the problem persists?
 Copyright Sage, 2010. Some rights reserved under creative commons license. Content on this site is licensed under a Creative Commons Attribution Share Alike 3.0 license.
 
                
                Copyright Sage, 2010. Some rights reserved under creative commons license. Content on this site is licensed under a Creative Commons Attribution Share Alike 3.0 license.