JspServletBuilder.setupDeployment() takes a Map as it's second argument, but line 85 of org.apache.jasper.compiler.JspConfig randomly casts this argument to a HashMap. The unnecessary cast should be removed, see the pull request on the github project for Jastow listed below.

The bug probably hasn't been found because most examples available online pass an empty HashMap to JspServletBuilder.setupDeployment().