ruby - Could not load : can't convert nil into String -

i want read logs after particular date. approach drop events previous date. try achieve this:

i dropping logs before june 1, 2015:

logstash config file:

input {          file{                 path => [                          "/var/log/rsyslog/**/*.log"                 ]         } }  filter {          grok {          match => ["path", "/var/log/rsyslog/(?<server>[^/]+)/%{year:year}-%{monthnum:month}-%{monthday:month_day}/(?<logtype>.*).log"]          }          if [year] < "2015" , [month] < "6" , [month_day] < "1" {                  drop { }          } 

my logstash.err file keeps printing this:

could not load : can't convert nil string 

any idea why?

one of 3 values - year, month or month_day nil. because regex not matching of lines in log file.