undefined method 'size' for nil (NoMethodError) on TIME_PARSER if MDTM command returns something different than a 213 response

When

resp = sendcmd("MDTM #{filename}")

returns something different than 213. (E.g., 550 Not a plain file, 502 Command not implemented)

Then the method mdtm

def mdtm(filename)
  resp = sendcmd("MDTM #{filename}")
  if resp.start_with?("213")
    return get_body(resp)
  end
  #
end

will return nil

and then the method mtime:

def mtime(filename, local = false)
  return TIME_PARSER.(mdtm(filename), local)
end

will raise an undefined method 'size' for nil (NoMethodError) on the TIME_PARSER, because value is nil

value = value[0, 97] + "..." if value.size > 100

Easy fix:

def mtime(filename, local = false)
  value = mdtm(filename)
  TIME_PARSER.(value, local) if value
end

Locate the mdtm and mtime methods and trace how a non-213 MDTM response reaches TIME_PARSER. Check the behavior for responses such as 550 and 502, then verify that mtime no longer raises NoMethodError when mdtm returns nil.