GSM - Measurement Report decoding problem

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Hello,

I am decoding GSM - MEASUREMENT Report and it shows incorrect/unknown value for IE "RXLEV-SUB-SERVING-CELL"

PDU hex: 06 15 bf 7f 7e 00 00 00 00 00 00 00 00 00 00 00 00 00

Decoded output:
    Measurement Results
        1... .... = BA-USED: 1
        .0.. .... = DTX-USED: DTX was not used
        ..11 1111 = RXLEV-FULL-SERVING-CELL: >= -48 dBm (63)
        0... .... = 3G-BA-USED: 0
        .1.. .... = MEAS-VALID: The measurement results are not valid
        ****RXLEV-SUB-SERVING-CELL: Unknown (127)****
        .111 .... = RXQUAL-FULL-SERVING-CELL: BER > 12.8%, Mean value 18.10% (7)
        .... 111. = RXQUAL-SUB-SERVING-CELL: BER > 12.8%, Mean value 18.10% (7)
        .... ...0  00.. .... = NO-NCELL-M: No neighbour cell measurement result (0)