diff --git a/scsh/rx/posixstr.scm b/scsh/rx/posixstr.scm
index a195ed7..bbd06e5 100644
--- a/scsh/rx/posixstr.scm
+++ b/scsh/rx/posixstr.scm
@@ -39,6 +39,11 @@
 ;;; 
 ;;;   - paren count in the returned string.
 ;;;
+;;;   [This is a newer description; is it correct?]
+;;;   - A vector mapping submatches (vector index 0 is submatch 1)
+;;;     to the paren for that submatch (the first paren is paren #1).
+;;;
+;;;   [This is my original description.]
 ;;;   - Vector of parens numbers used for submatching. The first paren is
 ;;;     numbered 1. #F means a dead submatch -- one we can tell statically
 ;;;     will never match anything.
@@ -199,7 +204,7 @@
 		  (values (string-append s1 s)
 			  2
 			  (+ pcount1 pcount)
-			  (vector-append (mapv (lambda (sm) (+ sm prev-smcount))
+			  (vector-append (mapv (lambda (p) (and p (+ p prev-pcount)))
 					       submatches1)
 					 submatches))))))