GroovyCastException with CompileStatic

@groovy.transform.CompileStatic
class Foo {
    CharSequence makeEnv( env, StringBuilder result = new StringBuilder() ) {
        if( env instanceof File ) {
            env = env.toPath()
        }
        if( env instanceof String && env.contains('=') ) {
            result << 'export ' << env << ';'
        }

        return result.toString()
    }
}

assert new Foo().makeEnv('X=1') == 'export X=1;'

Exception thrown

org.codehaus.groovy.runtime.typehandling.GroovyCastException: Cannot cast object 'X=1' with class 'java.lang.String' to class 'java.lang.Iterable'
	at Foo.makeEnv(ConsoleScript5:10)
	at Foo.makeEnv(ConsoleScript5:4)
	at Foo$makeEnv.call(Unknown Source)
	at ConsoleScript5.run(ConsoleScript5:20)