Null key for a Map not allowed in JSON

今天遇到一个奇葩问题，百思不得其解，望各位大佬给予解答：

这样运行的时候报错，

警告: Failed to write HTTP message: org.springframework.http.converter.HttpMessageNotWritableException: Could not write content: Null key for a Map not allowed in JSON (use a converting NullKeySerializer?); nested exception is com.fasterxml.jackson.core.JsonGenerationException: Null key for a Map not allowed in JSON (use a converting NullKeySerializer?)

这样的话，返回数据正常！baseResultMap对应的是上面的VoSysEnum.

求大佬们帮忙...