diff --git a/src/ls-twisted-products.lisp b/src/ls-twisted-products.lisp
index 21d00a3..bf7e7d4 100644
--- a/src/ls-twisted-products.lisp
+++ b/src/ls-twisted-products.lisp
@@ -372,7 +372,7 @@
      (multiple-value-bind (expn y-n+1 gp-n) (pop-first-absm n g-n)
         (declare
            (fixnum expn)
-           (type absm y-n+1 gp-n))
+           (type (or absm null) y-n+1 gp-n))
         (ecase expn
            (0 (zero-cmbn (1+ n)))
            (+1