Closure for creating key has an issue when used with @Transactional annotation

[jacobmarcus]

This issue is related to #49.

grails -v
Grails Version: 3.3.7
Groovy Version: 2.4.15
JVM Version: 1.8.0_222

The following service method will give the stacktrace below. Issue is with how the key is formed.
It works without the @transactional annotation. Version of cache plugin is 4.0.3.

    @Transactional(readOnly=true)
    @Cacheable(value='codes', key={code + '' + layer})
    def getCachedData(String code, String layer){
      return 'some code'
    }

Caused by: java.lang.NoSuchMethodError: java.lang.String.plus(Ljava/lang/CharSequence;)Ljava/lang/String;
at helloworld.SimpleService$__tt__getCachedData_closure4.doCall(SimpleService.groovy:89)
at grails.plugin.cache.CustomCacheKeyGenerator.generate(CustomCacheKeyGenerator.groovy:115)
at grails.gorm.transactions.GrailsTransactionTemplate$2.doInTransaction(GrailsTransactionTemplate.groovy:94)
at org.springframework.transaction.support.TransactionTemplate.execute(TransactionTemplate.java:133)