diff --git a/src/pkg/bufio/bufio.go b/src/pkg/bufio/bufio.go
index cd51585..4b69019 100644
--- a/src/pkg/bufio/bufio.go
+++ b/src/pkg/bufio/bufio.go
@@ -64,6 +64,8 @@
 	return NewReaderSize(rd, defaultBufSize)
 }
 
+var errNegativeRead = errors.New("bufio: reader returned negative count from Read")
+
 // fill reads a new chunk into the buffer.
 func (b *Reader) fill() {
 	// Slide existing data to beginning.
@@ -75,6 +77,9 @@
 
 	// Read new data.
 	n, e := b.rd.Read(b.buf[b.w:])
+	if n < 0 {
+		panic(errNegativeRead)
+	}
 	b.w += n
 	if e != nil {
 		b.err = e
@@ -282,6 +287,9 @@
 // of the line. The returned buffer is only valid until the next call to
 // ReadLine. ReadLine either returns a non-nil line or it returns an error,
 // never both.
+//
+// The text returned from ReadLine does not include the line end ("\r\n" or "\n").
+// No indication or error is given if the input ends without a final line end.
 func (b *Reader) ReadLine() (line []byte, isPrefix bool, err error) {
 	line, err = b.ReadSlice('\n')
 	if err == ErrBufferFull {