diff --git a/smtp/data.go b/smtp/data.go
index 7a59d75..ac73fa6 100644
--- a/smtp/data.go
+++ b/smtp/data.go
@@ -135,7 +135,7 @@ func (r *DataReader) Read(p []byte) (int, error) {
 		if len(r.buf) > 0 {
 			// Reject bare \r.
 			for i, c := range r.buf {
-				if c == '\r' && (i == len(r.buf) || r.buf[i+1] != '\n') {
+				if c == '\r' && (i == len(r.buf)-1 || r.buf[i+1] != '\n') {
 					r.badcrlf = true
 				}
 			}
diff --git a/smtp/data_test.go b/smtp/data_test.go
index f438b08..893271f 100644
--- a/smtp/data_test.go
+++ b/smtp/data_test.go
@@ -153,3 +153,9 @@ func (r *oneReader) Read(buf []byte) (int, error) {
 	r.buf = r.buf[1:]
 	return 1, nil
 }
+
+func FuzzReader(f *testing.F) {
+	f.Fuzz(func(t *testing.T, data string) {
+		io.ReadAll(NewDataReader(bufio.NewReader(strings.NewReader(data))))
+	})
+}