java.lang.StringIndexOutOfBoundsException: String index out of range: -7 ERROR?

Code:

java.lang.StringIndexOutOfBoundsException: String index out of range: -7
        at java.lang.String.substring(Unknown Source)
        at java.lang.String.substring(Unknown Source)
        at client.customCommand(client.java:12286)
        at client.parseIncomingPackets(client.java:22411)
        at client.packetProcess(client.java:18234)
        at client.process(client.java:18204)
        at PlayerHandler.process(PlayerHandler.java:137)
        at server.main(server.java:49)
TTKSCAPE V4Server [fatal] - exception

HOW Do i fix this? i tryed everything

it happens when we try to enter commands.

gets them every time

help pl0x