Cannot convert instance of class org.jruby.RubyTime to class java.lang.String"

filter {
date {
match => [ "timestamp", "yyyy-MM-dd HH:mm:ss " , "ISO8601" ]
target => "@timestamp"
}
}

I am using this filter in my conf file for feeding data from postgresql to logstash and from logstash to elasticsearch but i am getting error " cannot convert instance of class org.jruby.RubyTime to
class java.lang.String"," . As i have a column timestamp and logstash gives its own timestamp .. it is conflicting working of my code .. can anybody resolve this?