diff --git a/lib/scheme/extras.scm b/lib/scheme/extras.scm
index 1fd8a5f8..b7e7f491 100644
--- a/lib/scheme/extras.scm
+++ b/lib/scheme/extras.scm
@@ -176,9 +176,13 @@
   (let* ((start (if (pair? o) (car o) 0))
          (end (if (and (pair? o) (pair? (cdr o))) (cadr o) (vector-length from)))
          (limit (min end (+ start (- (vector-length to) at)))))
-    (do ((i at (+ i 1)) (j start (+ j 1)))
-        ((>= j limit))
-      (vector-set! to i (vector-ref from j)))))
+    (if (<= at start)
+        (do ((i at (+ i 1)) (j start (+ j 1)))
+            ((>= j limit))
+          (vector-set! to i (vector-ref from j)))
+        (do ((i (+ at (- end start 1)) (- i 1)) (j (- limit 1) (- j 1)))
+            ((< j start))
+          (vector-set! to i (vector-ref from j))))))
 
 (define (vector->string vec . o)
   (list->string (apply vector->list vec o)))
@@ -199,9 +203,13 @@
                   (cadr o)
                   (bytevector-length from)))
          (limit (min end (+ start (- (bytevector-length to) at)))))
-    (do ((i at (+ i 1)) (j start (+ j 1)))
-        ((>= j limit))
-      (bytevector-u8-set! to i (bytevector-u8-ref from j)))))
+    (if (<= at start)
+        (do ((i at (+ i 1)) (j start (+ j 1)))
+            ((>= j limit))
+          (bytevector-u8-set! to i (bytevector-u8-ref from j)))
+        (do ((i (+ at (- end start 1)) (- i 1)) (j (- limit 1) (- j 1)))
+            ((< j start))
+          (bytevector-u8-set! to i (bytevector-u8-ref from j))))))
 
 (define (bytevector-copy vec . o)
   (if (null? o)
@@ -223,9 +231,13 @@
   (let* ((start (if (pair? o) (car o) 0))
          (end (if (and (pair? o) (pair? (cdr o))) (cadr o) (string-length from)))
          (limit (min end (+ start (- (string-length to) at)))))
-    (do ((i at (+ i 1)) (j start (+ j 1)))
-        ((>= j limit))
-      (string-set! to i (string-ref from j)))))
+    (if (<= at start)
+        (do ((i at (+ i 1)) (j start (+ j 1)))
+            ((>= j limit))
+          (string-set! to i (string-ref from j)))
+        (do ((i (+ at (- end start 1)) (- i 1)) (j (- limit 1) (- j 1)))
+            ((< j start))
+          (string-set! to i (string-ref from j))))))
 
 (define truncate-quotient quotient)
 (define truncate-remainder remainder)
diff --git a/tests/r7rs-tests.scm b/tests/r7rs-tests.scm
index 6c490eec..1a1198fa 100644
--- a/tests/r7rs-tests.scm
+++ b/tests/r7rs-tests.scm
@@ -1096,6 +1096,12 @@
 (test "xx-xx"
     (let ((str (make-string 5 #\x))) (string-copy! str 2 "-----" 2 3) str))
 
+;; same source and dest
+(test "aabde"
+    (let ((str (string-copy "abcde"))) (string-copy! str 1 str 0 2) str))
+(test "abcab"
+    (let ((str (string-copy "abcde"))) (string-copy! str 3 str 0 2) str))
+
 (test-end)
 
 (test-begin "6.8 Vectors")
@@ -1169,6 +1175,12 @@
 (test #(1 2 c 4 5)
     (let ((vec (vector 1 2 3 4 5))) (vector-copy! vec 2 #(a b c d e) 2 3) vec))
 
+;; same source and dest
+(test #(1 1 2 4 5)
+    (let ((vec (vector 1 2 3 4 5))) (vector-copy! vec 1 vec 0 2) vec))
+(test #(1 2 3 1 2)
+    (let ((vec (vector 1 2 3 4 5))) (vector-copy! vec 3 vec 0 2) vec))
+
 (test-end)
 
 (test-begin "6.9 Bytevectors")
@@ -1219,6 +1231,16 @@
       (bytevector-copy! bv 2 #u8(6 7 8 9 10) 2 3)
       bv))
 
+;; same source and dest
+(test #u8(1 1 2 4 5)
+    (let ((bv (bytevector 1 2 3 4 5)))
+      (bytevector-copy! bv 1 bv 0 2)
+      bv))
+(test #u8(1 2 3 1 2)
+    (let ((bv (bytevector 1 2 3 4 5)))
+      (bytevector-copy! bv 3 bv 0 2)
+      bv))
+
 (test #u8() (bytevector-append #u8()))
 (test #u8() (bytevector-append #u8() #u8()))
 (test #u8(0 1 2) (bytevector-append #u8() #u8(0 1 2)))