A speaking error message, maybe including the class name, for which a transformer is missing would be better.

java.lang.NullPointerException
at org.infinispan.query.backend.KeyTransformationHandler.getTransformerClassFromAnnotation(KeyTransformationHandler.java:204)
at org.infinispan.query.backend.KeyTransformationHandler.getTransformerClass(KeyTransformationHandler.java:196)
at org.infinispan.query.backend.KeyTransformationHandler.getTransformer(KeyTransformationHandler.java:189)
at org.infinispan.query.backend.KeyTransformationHandler.keyToString(KeyTransformationHandler.java:154)
at org.infinispan.query.backend.QueryInterceptor.keyToString(QueryInterceptor.java:310)
at org.infinispan.query.backend.QueryInterceptor.addToIndexes(QueryInterceptor.java:220)