String index out of range: 391949 when try to minify combined css output

[Rocks360]

Hello,

when I try to minify my style.css and also bootstrap.css and font-awesome.css to one file with the Console command "./gradlew minifyCss " I get the following error:

:combineCss UP-TO-DATE
:minifyCss FAILED

FAILURE: Build failed with an exception.

* What went wrong:
Execution failed for task ':minifyCss'.
> String index out of range: 391949

* Try:
Run with --stacktrace option to get the stack trace. Run with --info or --debug option to get more log output.

BUILD FAILED

Total time: 0.63 secs

combine all css to one file works. Just to minify them not.(look line one above form console output)