[pig] UI treats numbered variables as parameters

Here is the part of script:

event_raw = load '/user/hdfs/kill_me/xvlr_data' using com.twitter.elephantbird.pig.load.SequenceFileLoader('-c com.twitter.elephantbird.pig.util.NullWritableConverter', '-c com.twitter.elephantbird.pig.util.TextConverter');
event_tuple = foreach event_raw generate STRSPLIT(value, '\t', 13).($0, $1, $3, $4, $5, $11) as e: (msisdn: chararray, imei: chararray, lac: chararray, cid: chararray, event_type: chararray, ts: chararray);

Pig web UI promts for parameters when I do try to click "Run".
All these $0, $1 are not parameters, they are numbered fields

Script execution is failed.

013-06-24 12:56:21,959 [main] INFO org.apache.pig.tools.parameters.ParameterSubstitutionPreprocessor - The parameter: "0=" cannot be parsed by Pig. Please double check it
2013-06-24 12:56:21,959 [main] INFO org.apache.pig.tools.parameters.ParameterSubstitutionPreprocessor - The parameter: "0=" cannot be parsed by Pig. Please double check it
2013-06-24 12:56:21,960 [main] INFO org.apache.pig.tools.parameters.ParameterSubstitutionPreprocessor - Parser give the follow error message:
2013-06-24 12:56:21,960 [main] INFO org.apache.pig.tools.parameters.ParameterSubstitutionPreprocessor - Parser give the follow error message:
2013-06-24 12:56:21,960 [main] INFO org.apache.pig.tools.parameters.ParameterSubstitutionPreprocessor - Encountered " <OTHER> "0= "" at line 1, column 1.
Was expecting one of:
<EOF>
<IDENTIFIER> ...
<COMMENT> ...

2013-06-24 12:56:21,960 [main] INFO org.apache.pig.tools.parameters.ParameterSubstitutionPreprocessor - Encountered " <OTHER> "0= "" at line 1, column 1.
Was expecting one of:
<EOF>
<IDENTIFIER> ...
<COMMENT> ...