diff --git a/tb/axis_replay_buffer.py b/tb/axis_replay_buffer.py
index 63e0ba8..7002e9e 100644
--- a/tb/axis_replay_buffer.py
+++ b/tb/axis_replay_buffer.py
@@ -10,6 +10,28 @@ from .util import ClockEnable, lookahead, timeout
 
 BUF_SIZE = 54
 
+async def send_packet(signals, packet, ratio=1):
+    for val, last in lookahead(packet):
+        if 'err' in signals:
+            if val is None:
+                signals['data'].value = 0
+                signals['err'].value = 1
+            else:
+                signals['data'].value = val
+                signals['err'].value = 0
+        else:
+            signals['data'].value = val
+        signals['valid'].value = 1
+        signals['last'].value = last
+        await RisingEdge(signals['clk'])
+        while True:
+            await FallingEdge(signals['clk'])
+            if signals['ready'].value:
+                break
+        signals['valid'].value = 0
+        if ratio != 1:
+            await ClockCycles(signals['clk'], ratio - 1, rising=False)
+
 @timeout(30, 'us')
 async def test_replay(buf, in_ratio, out_ratio):
     buf.s_axis_valid.value = 0
@@ -31,17 +53,13 @@ async def test_replay(buf, in_ratio, out_ratio):
 
     async def send():
         for packet in packets:
-            for val, last in lookahead(packet):
-                buf.s_axis_data.value = val
-                buf.s_axis_valid.value = 1
-                buf.s_axis_last.value = last
-                while True:
-                    await FallingEdge(buf.clk)
-                    if buf.s_axis_ready.value:
-                        break
-                buf.s_axis_valid.value = 0
-                if in_ratio != 1:
-                    await ClockCycles(buf.clk, in_ratio - 1, rising=False)
+            await send_packet({
+                'clk': buf.clk,
+                'data': buf.s_axis_data,
+                'valid': buf.s_axis_valid,
+                'last': buf.s_axis_last,
+                'ready': buf.s_axis_ready,
+            }, packet, in_ratio)
 
     async def recv(packet):
         async def handshake():