diff --git a/srfi-13.scm b/srfi-13.scm
index 7b16153..f45ebed 100644
--- a/srfi-13.scm
+++ b/srfi-13.scm
@@ -1066,7 +1066,7 @@
   (let-optionals* criteria+start+end ((criteria char-set:whitespace) rest)
     (let-string-start+end (start end) string-trim-right s rest
       (cond ((string-skip-right s criteria start end) =>
-	     (lambda (i) (%substring/shared s 0 (+ 1 i))))
+	     (lambda (i) (%substring/shared s (min start (+ i 1)) (+ 1 i))))
 	    (else "")))))
 
 (define (string-trim-both s . criteria+start+end)
@@ -1208,17 +1208,17 @@
   (let-string-start+end (start end) string-index-right str maybe-start+end
     (cond ((char? criteria)
 	   (let lp ((i (- end 1)))
-	     (and (>= i 0)
+	     (and (>= i start)
 		  (if (char=? criteria (string-ref str i)) i
 		      (lp (- i 1))))))
 	  ((char-set? criteria)
 	   (let lp ((i (- end 1)))
-	     (and (>= i 0)
+	     (and (>= i start)
 		  (if (char-set-contains? criteria (string-ref str i)) i
 		      (lp (- i 1))))))
 	  ((procedure? criteria)
 	   (let lp ((i (- end 1)))
-	     (and (>= i 0)
+	     (and (>= i start)
 		  (if (criteria (string-ref str i)) i
 		      (lp (- i 1))))))
 	  (else (##sys#error 'string-index-right "Second param is neither char-set, char, or predicate procedure."
@@ -1250,19 +1250,19 @@
   (let-string-start+end (start end) string-skip-right str maybe-start+end
     (cond ((char? criteria)
 	   (let lp ((i (- end 1)))
-	     (and (>= i 0)
+	     (and (>= i start)
 		  (if (char=? criteria (string-ref str i))
 		      (lp (- i 1))
 		      i))))
 	  ((char-set? criteria)
 	   (let lp ((i (- end 1)))
-	     (and (>= i 0)
+	     (and (>= i start)
 		  (if (char-set-contains? criteria (string-ref str i))
 		      (lp (- i 1))
 		      i))))
 	  ((procedure? criteria)
 	   (let lp ((i (- end 1)))
-	     (and (>= i 0)
+	     (and (>= i start)
 		  (if (criteria (string-ref str i)) (lp (- i 1))
 		      i))))
 	  (else (##sys#error 'string-skip-right "CRITERIA param is neither char-set or char."