diff --git a/library/DIAMETER_Emulation.ttcn b/library/DIAMETER_Emulation.ttcn
index e7481ca..69107d4 100644
--- a/library/DIAMETER_Emulation.ttcn
+++ b/library/DIAMETER_Emulation.ttcn
@@ -37,6 +37,7 @@
 import from DIAMETER_Templates all;
 import from Osmocom_Types all;
 import from IPL4asp_Types all;
+import from TCCConversion_Functions all;
 import from Native_Functions all;
 
 type hexstring IMSI;
@@ -297,7 +298,13 @@
 		return str2hex(oct2char(grp[1].avp.avp_data.avp_DCC_NONE_Subscription_Id_Data));
 	} else {
 		var octetstring imsi_oct := valueof(imsi_avp.avp_data.avp_BASE_NONE_User_Name);
-		return str2hex(oct2char(imsi_oct));
+		var charstring imsi_str := oct2char(imsi_oct);
+		/* Username may be a NAI instead of IMSI: "<IMSI>@nai.epc.mnc<MNC>.mcc<MCC>.3gppnetwork.org" */
+		var integer pos := f_strstr(imsi_str, "@");
+		if (pos != -1) {
+			imsi_str := substr(imsi_str, 0, pos);
+		}
+		return str2hex(imsi_str);
 	}
 }