diff --git a/parser.go b/parser.go
index 8eeb450..c972aea 100644
--- a/parser.go
+++ b/parser.go
@@ -188,6 +188,9 @@ func (d *decoder) Decode(v *packet) error {
 	if err != nil {
 		return err
 	}
+	if t < '0' || t > '0'+byte(BinaryAck) {
+		return fmt.Errorf("invalid packet type %q", t)
+	}
 	v.Type = packetType(t - '0')
 
 	if v.Type == BinaryEvent || v.Type == BinaryAck {
diff --git a/parser_test.go b/parser_test.go
index 2d6ac23..2aae731 100644
--- a/parser_test.go
+++ b/parser_test.go
@@ -28,6 +28,24 @@ func TestPacketType(t *testing.T) {
 	})
 }
 
+func TestDecodeRejectsInvalidPacketType(t *testing.T) {
+	for _, input := range []string{"/", "7", "9[]"} {
+		t.Run(input, func(t *testing.T) {
+			saver := &FrameSaver{
+				data: []FrameData{{
+					Buffer: bytes.NewBufferString(input),
+					Type:   engineio.MessageText,
+				}},
+			}
+
+			var p packet
+			if err := newDecoder(saver).Decode(&p); err == nil {
+				t.Fatalf("Decode(%q) error = nil, want invalid packet type error", input)
+			}
+		})
+	}
+}
+
 func TestParser(t *testing.T) {
 	p := packet{}
 	var decodeData any