diff --git a/compiler/bit-util.lisp b/compiler/bit-util.lisp
index 6aec56fd3d9f92e5fb85dce40532ab787aa8b686..4fe910e31c2a867898766a142dd7f27dfe28dd2e 100644
--- a/compiler/bit-util.lisp
+++ b/compiler/bit-util.lisp
@@ -47,4 +47,4 @@
 ;;;
 (defun bit-vector-copy (vec)
   (declare (type simple-bit-vector vec))
-  (bit-ior vec vec))
+  (bit-ior vec vec (make-array (length vec) :element-type 'bit)))