diff --git a/tb/gmii_ep.py b/tb/gmii_ep.py
index 528be9ab2..eccbeb398 100644
--- a/tb/gmii_ep.py
+++ b/tb/gmii_ep.py
@@ -88,6 +88,8 @@ def GMIISource(clk, rst,
                fifo=None,
                name=None):
 
+    assert len(txd) == 8
+
     @instance
     def logic():
         frame = None
@@ -141,6 +143,8 @@ def GMIISink(clk, rst,
              fifo=None,
              name=None):
 
+    assert len(rxd) == 8
+
     @instance
     def logic():
         frame = None
diff --git a/tb/xgmii_ep.py b/tb/xgmii_ep.py
index 5f29af571..cca98fb6a 100644
--- a/tb/xgmii_ep.py
+++ b/tb/xgmii_ep.py
@@ -109,6 +109,8 @@ def XGMIISource(clk, rst,
                 fifo=None,
                 name=None):
 
+    assert len(txd) == 64
+
     @instance
     def logic():
         frame = None
@@ -210,6 +212,8 @@ def XGMIISink(clk, rst,
               fifo=None,
               name=None):
 
+    assert len(rxd) == 64
+
     @instance
     def logic():
         frame = None