--- ../nnmairix_0.4.el	2008-01-28 20:36:23.000000000 +0100
+++ nnmairix.el	2008-01-28 20:30:56.000000000 +0100
@@ -1025,10 +1025,10 @@
 	    (when corr
 		  (setq article (+ (mail-header-number cur) numc))
 		  (mail-header-set-number cur article))
-	    (setq xref (replace-regexp-in-string
-			(format "[ \t]%s:[0-9]+" backendgroup)
-			(format " %s:%d" mairixgroup article)
-			(mail-header-xref cur) t t))
+	    (setq xref (mail-header-xref cur))
+	    (when (and (stringp xref)
+		       (string-match (format "[ \t]%s:[0-9]+" backendgroup) xref))
+	      (setq xref (replace-match (format " %s:%d" mairixgroup article) t nil xref)))
 	    (mail-header-set-xref cur xref)
 	    (set-buffer buf)
 	    (nnheader-insert-nov cur)