diff --git a/compiler/ir1opt.lisp b/compiler/ir1opt.lisp
index 6244e0ecd9a16d54a0b2290a5f42db5c69509f43..d4649aba0dce3d1be4ec674b9a5fbc9fbf22aaf2 100644
--- a/compiler/ir1opt.lisp
+++ b/compiler/ir1opt.lisp
@@ -46,6 +46,7 @@
 ;;;
 (proclaim '(function continuation-value (continuation) t))
 (defun continuation-value (cont)
+  (assert (constant-continuation-p cont))
   (constant-value (ref-leaf (continuation-use cont))))