diff --git a/cocotbext/axi/axi_master.py b/cocotbext/axi/axi_master.py
index 89b9b84..054c8b5 100644
--- a/cocotbext/axi/axi_master.py
+++ b/cocotbext/axi/axi_master.py
@@ -442,6 +442,9 @@ class AxiMasterRead(object):
                 raise Exception("Token is not unique")
             self.active_tokens.add(token)
 
+        if length < 0:
+            raise ValueError("Read length must be positive")
+
         if arid is None or arid < 0:
             arid = None
         elif arid > self.id_count: