diff --git a/app/Interpreter.hs b/app/Interpreter.hs
index 0107bf5..5d1e2ab 100644
--- a/app/Interpreter.hs
+++ b/app/Interpreter.hs
@@ -242,10 +242,12 @@ unify (Number a) (Number b)
 unify (Struct a) VoidRef = do
   uNext
   cur . gol %= (replicate (arity a) (U VoidRef) ++)
+  cur . unis += arity a
   continue
 unify VoidRef (Struct a) = do
   uNext
   cur . hed %= (replicate (arity a) (U VoidRef) ++)
+  cur . unis += arity a
   continue
 unify (Struct a) (Struct b)
   | a == b = do