This error is because swfc assign an nonexistent charecter ID for slave.swf's document class Slave. This problem can be located by swfdump master.swf:
[HEADER] File version: 9
[HEADER] File is zlib compressed. Ratio: 57%
[HEADER] File size: 1213
[HEADER] Frame rate: 15.000000
[HEADER] Frame count: 1
[HEADER] Movie width: 200.00
[HEADER] Movie height: 200.00
[045] 4 FILEATTRIBUTES usenetwork as3 symbolclass
[009] 3 SETBACKGROUNDCOLOR (00/00/00)
[052] 301 DOABC
[04c] 11 SYMBOLCLASS
exports 0000 as "Master"
[04d] 456 METADATA
[041] 4 SCRIPTLIMITS
[029] 26 SERIALNUMBER
[04c] 10 SYMBOLCLASS
exports 0002 as "Slave" # Should be 0001 here
[027] 4 DEFINESPRITE defines id 0001
[02b] 6 FRAMELABEL "Slave"
[052] 287 DOABC "frame1", lazy load
[001] 0 SHOWFRAME 1 (00:00:00,000) (label "Slave")
[000] 0 END
[028] 8 NAMECHARACTER adds information to id 0001 "slave"
[038] 10 EXPORTASSETS
exports 0001 as "slave"
[01a] 11 PLACEOBJECT2 places id 0001 at depth 0001 name "slave"
[001] 0 SHOWFRAME 1 (00:00:00,000)
[000] 0 END
The version I use is swftools 2012-10-15-1307