diff --git a/library/IuUP_Emulation.ttcn b/library/IuUP_Emulation.ttcn
index 56384ee70..b35602cef 100644
--- a/library/IuUP_Emulation.ttcn
+++ b/library/IuUP_Emulation.ttcn
@@ -143,7 +143,7 @@ function f_IuUP_Em_tx_encap(inout IuUP_Entity st, in octetstring payload) return
 			} else {
 				pdu := valueof(ts_IuUP_Type1(st.tx_next_frame_nr, 0, payload));
 			}
-			st.tx_next_frame_nr := st.tx_next_frame_nr + 1;
+			st.tx_next_frame_nr := (st.tx_next_frame_nr + 1) mod 16;
 		}
 	}
 	if (isvalue(pdu)) {