mvn release:prepare fails with 'Cannot prepare the release because you have local modifications'

Can't perform maven release prepare.

[INFO] ------------------------------------------------------------------------
[ERROR] BUILD FAILURE
[INFO] ------------------------------------------------------------------------
[INFO] Cannot prepare the release because you have local modifications :
[build-number.txt:unknown]

[INFO] ------------------------------------------------------------------------
[INFO] For more information, run Maven with the -e switch

Maven job parameters:

clean release:prepare -Darguments='-Dmaven.test.skip=true' --batch-mode

Also tried
clean release:prepare -Darguments='-Dmaven.test.skip=true'