diff --git a/library/OSMUX_Emulation.ttcn b/library/OSMUX_Emulation.ttcn
index c6999de..4538fdf 100644
--- a/library/OSMUX_Emulation.ttcn
+++ b/library/OSMUX_Emulation.ttcn
@@ -374,12 +374,20 @@
 				log2str("No Space in TxHandleList for ", hdl.cid));
 }
 
+/* Generate correctly-sized AMR payload based on amr_ft, using tx_fixed_payload as a fill pattern. */
 function f_osmux_gen_expected_rx_rtp_payload(INT4b amr_ft, octetstring tx_fixed_payload) return octetstring {
+	var integer payload_len_bits;
 	var integer payload_len;
+	var bitstring payload_truncated_bits;
 	var octetstring payload_truncated;
-	var integer i;
+
+	/* Generate the AMR payload as a bitstring, since some formats don't end up in octet boundary: */
+	payload_len_bits := f_amrft_payload_bits_len(amr_ft);
+	payload_truncated_bits := substr(oct2bit(tx_fixed_payload), 0, payload_len_bits);
+
+	/* Now convert it as an octet string filling final padding with zeroes: */
 	payload_len := f_amrft_payload_len(amr_ft);
-	payload_truncated := substr(tx_fixed_payload, 0, payload_len);
+	payload_truncated := bit2oct(f_pad_bit(payload_truncated_bits, payload_len*8, '0'B));
 	return payload_truncated;
 }