diff --git a/src/bufio/bufio.go b/src/bufio/bufio.go
index 46df6192280b8e..ffb278ad9e2d00 100644
--- a/src/bufio/bufio.go
+++ b/src/bufio/bufio.go
@@ -128,6 +128,9 @@ func (b *Reader) Peek(n int) ([]byte, error) {
 		return nil, ErrNegativeCount
 	}
 
+	b.lastByte = -1
+	b.lastRuneSize = -1
+
 	for b.w-b.r < n && b.w-b.r < len(b.buf) && b.err == nil {
 		b.fill() // b.w-b.r < len(b.buf) => buffer is not full
 	}
diff --git a/src/bufio/bufio_test.go b/src/bufio/bufio_test.go
index 34d70312f7bc0e..f7a0682e7033dd 100644
--- a/src/bufio/bufio_test.go
+++ b/src/bufio/bufio_test.go
@@ -285,6 +285,24 @@ func TestUnreadRune(t *testing.T) {
 	}
 }
 
+func TestNoUnreadRuneAfterPeek(t *testing.T) {
+	br := NewReader(strings.NewReader("example"))
+	br.ReadRune()
+	br.Peek(1)
+	if err := br.UnreadRune(); err == nil {
+		t.Error("UnreadRune didn't fail after Peek")
+	}
+}
+
+func TestNoUnreadByteAfterPeek(t *testing.T) {
+	br := NewReader(strings.NewReader("example"))
+	br.ReadByte()
+	br.Peek(1)
+	if err := br.UnreadByte(); err == nil {
+		t.Error("UnreadByte didn't fail after Peek")
+	}
+}
+
 func TestUnreadByte(t *testing.T) {
 	segments := []string{"Hello, ", "world"}
 	r := NewReader(&StringReader{data: segments})