SNMP check for queue data rate

On my SRX devices, I'm successfully monitoring CoS queue data rates using the jnxCosQstatTxedByteRate OID, but a query on EX devices always returns a rate of zero, even though the xmit byte count is increasing.

For example:

> show snmp mib walk jnxCosQstatTxedBytes.749
jnxCosQstatTxedBytes.749.0 = 49635243589049

> show snmp mib walk jnxCosQstatTxedByteRate.749
jnxCosQstatTxedByteRate.749.0 = 0

I'm 100% certain the current byte rate is much greater than zero based on the topology and other SRX monitors so I'm wondering if the EX simply doesn't support queue rate tracking, but I haven't turned up any documentation yet to confirm.  Does anyone have any insight?

Thanks