src/pkg/bufio/bufio.go      | 2 +-
 src/pkg/bufio/bufio_test.go | 9 +++++++++

diff --git a/src/pkg/bufio/bufio.go b/src/pkg/bufio/bufio.go
index 8c951903a1767d76b7c1237ce3357eb9cad66efc..70caf5dae81412e14d5224b9244687d438eb56ca 100644
--- a/src/pkg/bufio/bufio.go
+++ b/src/pkg/bufio/bufio.go
@@ -226,7 +226,7 @@ // the buffer was not a ReadRune, UnreadRune returns an error.  (In this
 // regard it is stricter than UnreadByte, which will unread the last byte
 // from any read operation.)
 func (b *Reader) UnreadRune() os.Error {
-	if b.lastRuneSize < 0 {
+	if b.lastRuneSize < 0 || b.r == 0 {
 		return ErrInvalidUnreadRune
 	}
 	b.r -= b.lastRuneSize
diff --git a/src/pkg/bufio/bufio_test.go b/src/pkg/bufio/bufio_test.go
index fb0ed045aebc4bd6b515ec18cfba26b97baa2c50..fe04b91691e908793dfdcda1a59a3de4e0bddd2e 100644
--- a/src/pkg/bufio/bufio_test.go
+++ b/src/pkg/bufio/bufio_test.go
@@ -564,3 +564,12 @@ 	if _, err := buf.Peek(1); err != os.EOF {
 		t.Fatalf("want EOF got %v", err)
 	}
 }
+
+func TestPeekThenUnreadRune(t *testing.T) {
+	// This sequence used to cause a crash.
+	r := NewReader(strings.NewReader("x"))
+	r.ReadRune()
+	r.Peek(1)
+	r.UnreadRune()
+	r.ReadRune() // Used to panic here
+}