diff --git a/bts/BTS_Tests.ttcn b/bts/BTS_Tests.ttcn
index 5637f63..fa4e426 100644
--- a/bts/BTS_Tests.ttcn
+++ b/bts/BTS_Tests.ttcn
@@ -5829,7 +5829,9 @@
 
 /* Send IMM.ASS from PCU for PCH; check it appears on Um side */
 testcase TC_pcu_data_req_imm_ass_pch() runs on test_CT {
-	var octetstring imm_ass := f_rnd_octstring(23);
+	var octetstring req_ref := f_rnd_octstring(3);
+	var octetstring tlli := f_rnd_octstring(4);
+	var octetstring imm_ass := '2d063f300fc364'O & req_ref & '0000dc'O & tlli & '00232b2b2b2b'O;
 	f_init_with_pcuif();
 	f_init_l1ctl();
 	f_l1_tune(L1CTL);