diff --git a/src/io/io.go b/src/io/io.go
index 86710ed6f3..27482de62e 100644
--- a/src/io/io.go
+++ b/src/io/io.go
@@ -335,7 +335,7 @@ func ReadFull(r Reader, buf []byte) (n int, err error) {
 // If dst implements the ReaderFrom interface,
 // the copy is implemented using it.
 func CopyN(dst Writer, src Reader, n int64) (written int64, err error) {
-	written, err = copyN(dst, src, n)
+	written, err = Copy(dst, LimitReader(src, n))
 	if written == n {
 		return n, nil
 	}
@@ -346,55 +346,6 @@ func CopyN(dst Writer, src Reader, n int64) (written int64, err error) {
 	return
 }
 
-// copyN copies n bytes (or until an error) from src to dst.
-// It returns the number of bytes copied and the earliest
-// error encountered while copying.
-//
-// If dst implements the ReaderFrom interface,
-// the copy is implemented using it.
-func copyN(dst Writer, src Reader, n int64) (int64, error) {
-	// If the writer has a ReadFrom method, use it to do the copy.
-	if rt, ok := dst.(ReaderFrom); ok {
-		return rt.ReadFrom(LimitReader(src, n))
-	}
-
-	l := 32 * 1024 // same size as in copyBuffer
-	if n < int64(l) {
-		l = int(n)
-	}
-	buf := make([]byte, l)
-
-	var written int64
-	for n > 0 {
-		if n < int64(len(buf)) {
-			buf = buf[:n]
-		}
-
-		nr, errR := src.Read(buf)
-		if nr > 0 {
-			n -= int64(nr)
-			nw, errW := dst.Write(buf[:nr])
-			if nw > 0 {
-				written += int64(nw)
-			}
-			if errW != nil {
-				return written, errW
-			}
-			if nr != nw {
-				return written, ErrShortWrite
-			}
-		}
-
-		if errR != nil {
-			if errR != EOF {
-				return written, errR
-			}
-			return written, nil
-		}
-	}
-	return written, nil
-}
-
 // Copy copies from src to dst until either EOF is reached
 // on src or an error occurs. It returns the number of bytes
 // copied and the first error encountered while copying, if any.
@@ -434,8 +385,16 @@ func copyBuffer(dst Writer, src Reader, buf []byte) (written int64, err error) {
 	if rt, ok := dst.(ReaderFrom); ok {
 		return rt.ReadFrom(src)
 	}
+	size := 32 * 1024
+	if l, ok := src.(*LimitedReader); ok && int64(size) > l.N {
+		if l.N < 1 {
+			size = 1
+		} else {
+			size = int(l.N)
+		}
+	}
 	if buf == nil {
-		buf = make([]byte, 32*1024)
+		buf = make([]byte, size)
 	}
 	for {
 		nr, er := src.Read(buf)
diff --git a/src/io/io_test.go b/src/io/io_test.go
index e81065c13d..0e4ce61240 100644
--- a/src/io/io_test.go
+++ b/src/io/io_test.go
@@ -32,6 +32,21 @@ func TestCopy(t *testing.T) {
 	}
 }
 
+func TestCopyNegative(t *testing.T) {
+	rb := new(Buffer)
+	wb := new(Buffer)
+	rb.WriteString("hello")
+	Copy(wb, &LimitedReader{R: rb, N: -1})
+	if wb.String() != "" {
+		t.Errorf("Copy on LimitedReader with N<0 copied data")
+	}
+
+	CopyN(wb, rb, -1)
+	if wb.String() != "" {
+		t.Errorf("CopyN with N<0 copied data")
+	}
+}
+
 func TestCopyBuffer(t *testing.T) {
 	rb := new(Buffer)
 	wb := new(Buffer)