Only one active ctg on an instance allowed

I'm trying to set a DSCP value for an HTTP response based on the MIME type returned from the application services. Things are being tag correctly, but occasionally, I get the following message in the LTM log. Does anyone know how to decipher this to tell me the underlying issue? What is a "ctg"? Category? and what is the "instance" referring to?

TCL error: /Common/SND-BWC-Rule  - Only one active ctg on an instance allowed.: unable to set bwc app policy 'Media-Category' (line 1) invoked from within "BWC::color set AODefault-BWC Media-Category"

iRule

when CLIENT_ACCEPTED {
     log local0. "attached Media-BWC [IP::remote_addr]:[TCP::remote_port]"
    BWC::policy attach AODefault-BWC [IP::remote_addr]:[TCP::remote_port]
}

when HTTP_RESPONSE_RELEASE {
    if { [HTTP::header Content-Type] equals "image/jpeg" } {
         log local0. "colored  Media-BWC Media-Category"
        BWC::color set AODefault-BWC Media-Category
    }
}

Should I just wrap the BWC::color in a TCL exception check to log and ignore this error? Is there a better way to handle or check if a category has already been assigned?