diff --git a/src/io/io.go b/src/io/io.go
index 28dab08e46..86710ed6f3 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 = Copy(dst, LimitReader(src, n))
+	written, err = copyN(dst, src, n)
 	if written == n {
 		return n, nil
 	}
@@ -346,6 +346,55 @@ 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.