diff --git a/pkg/arvo/sys/hoon.hoon b/pkg/arvo/sys/hoon.hoon
index db93638941..1ca16b1842 100644
--- a/pkg/arvo/sys/hoon.hoon
+++ b/pkg/arvo/sys/hoon.hoon
@@ -217,6 +217,7 @@
   ::  computes the axis of {b} within axis {a}.
   |=  [a=@ b=@]
   ?<  =(0 a)
+  ?<  =(0 b)
   ::  a composed axis
   ^-  @
   ?-  b