How to remove part of name in multiple fields with Ruby in logstash?

[alytkowski]

@Badger

        event.to_hash.each { |k,v|
            if k =~ "^prometheus.metrics.ems"
                newK = k.sub(/^prometheus.metrics.ems/, "")
                event.remove(k)
                event.set(newK, v)
            end
        }

Unfortunately this gives out below error in the logs:
[ERROR][logstash.filters.ruby ][main] Ruby exception occurred: type mismatch: String given