diff --git a/convert.lisp b/convert.lisp
index c2683ae..63024d6 100644
--- a/convert.lisp
+++ b/convert.lisp
@@ -633,7 +633,7 @@ when NAME is not NIL."
             ;; the same name and collect their respective numbers
             (loop for name in reg-names
                   for reg-index from 0
-                  when (string= name backref-name)
+                  when (and (stringp name) (string= name backref-name))
                   ;; NOTE: REG-NAMES stores register names in reversed
                   ;; order REG-NUM contains number of (any) registers
                   ;; seen so far; 1- will be done later
diff --git a/test/simple b/test/simple
index 8c86c34..dba5104 100644
--- a/test/simple
+++ b/test/simple
@@ -364,3 +364,13 @@ characters if there's a match."
            (regex-replace-all (create-scanner "\\p{even}") "abcd" "+")
            (regex-replace-all (create-scanner "\\p{true}") "abcd" "+")))
    '("+b+d" "a+c+" "++++")))
+
+(handler-case
+    (progn
+      (scan '(:sequence
+              (:register "f")
+              (:register "o")
+              (:back-reference "NIL"))
+            "foo")
+      nil)
+  (error () t))