Layer: Unexpected error in replaceDataSource in SDE_WORKSPACE

JohnCuthbertson1

ESRI 10.5

I have an MXD file with a single layer pointing to an SDE Feature class and I want to point to an SDE Feature class with a different name

C:\Temp\jc_mxd\AppsInternal\simple.mxd
Layer name =SDI.MTD.LAYER
dataSource =c:\temp\SDI.sde\SDI.MTD.LAYER
Name           :SDI.MTD.LAYER
baseName       :SDI.MTD.LAYER
CatalogPath    :C:\Temp\SDI.sde\SDI.MTD.LAYER
dataElementType:DEFeatureClass
DataType       :FeatureClass
extension      :
file           :SDI.MTD.LAYER
path           :C:\Temp\SDI.sde
Traceback (most recent call last):
  File "mxd5.py", line 22, in <module>
    lyr.replaceDataSource ("c:\temp\SDI.sde","SDE_WORKSPACE","MTD.NEW" )