diff --git a/q931.c b/q931.c
index 4dac18d..b7f6a47 100755
--- a/q931.c
+++ b/q931.c
@@ -1935,7 +1935,7 @@ int q931_receive(struct pri *pri, q931_h *h, int len)
 		/* This is the weird maintenance stuff.  We majorly
 		   KLUDGE this by changing byte 4 from a 0xf (SERVICE) 
 		   to a 0x7 (SERVICE ACKNOWLEDGE) */
-		h->raw[3] -= 0x8;
+		h->raw[h->crlen + 2] -= 0x8;
 		q931_xmit(pri, h, len, 1);
 		return 0;
 	} else if (h->pd != Q931_PROTOCOL_DISCRIMINATOR) {