diff --git a/openpower/ext_interfaces/test_vmi.robot b/openpower/ext_interfaces/test_vmi.robot
index 91d43b6..7203f01 100644
--- a/openpower/ext_interfaces/test_vmi.robot
+++ b/openpower/ext_interfaces/test_vmi.robot
@@ -42,7 +42,7 @@
 
 ${default}                0.0.0.0
 ${default_ipv6addr}       ::
-${prefix_length}          64
+${prefix_length}          ${64}
 
 
 *** Test Cases ***