diff --git a/libs/openzap/src/isdn/Q931ie.c b/libs/openzap/src/isdn/Q931ie.c
index 23e4f010ed..d3cea9936f 100644
--- a/libs/openzap/src/isdn/Q931ie.c
+++ b/libs/openzap/src/isdn/Q931ie.c
@@ -1029,7 +1029,12 @@ L3INT Q931Uie_Cause(Q931_TrunkInfo_t *pTrunk, Q931mes_Generic *pMsg, L3UCHAR * I
     pie->Value = IBuf[Octet+Off] & 0x7f;
 	Octet++;
 
-    Q931IESizeTest(Q931E_CAUSE);
+	/* Consume optional Diagnostic bytes */
+	while (Q931MoreIE()) {
+		Off++;
+	};
+
+	Q931IESizeTest(Q931E_CAUSE);
 
     Q931SetIE(*pIE, *OOff);