diff --git a/ws/client.go b/ws/client.go
index 2561085..999555b 100644
--- a/ws/client.go
+++ b/ws/client.go
@@ -144,8 +144,13 @@ func clientReader(c *client){
 
 		/*** Parse message ***/
 		msg, err := readMessage(c.io.reader)
+		if err == UnmaskedFrameErr {
+			errorCode = PROTOCOL_ERR
+			clientAck = false
+			break
+		}
+
 		if err != nil {
-			// fmt.Printf(" [reader] %s\n", err)
 			break
 		}
 
diff --git a/ws/message.go b/ws/message.go
index 58b6f5f..3060d5a 100644
--- a/ws/message.go
+++ b/ws/message.go
@@ -1,11 +1,13 @@
 package ws
 
 import (
+	"fmt"
 	"io"
-	"net"
 	"encoding/binary"
 )
 
+var UnmaskedFrameErr = fmt.Errorf("Received unmasked frame")
+
 // Maximum Header Size  = Final/OpCode + isMask/Length + Length + Mask
 const maximumHeaderSize = 1 + 1 + 8 + 4
 
@@ -37,10 +39,10 @@ const (
 
 // Represents a websocket message
 type Message struct {
-	Type  MessageType
-	Data  []byte
-	Size  uint
 	Final bool
+	Type  MessageType
+	Size  uint
+	Data  []byte
 }
 
 
@@ -141,6 +143,11 @@ func readMessage(reader io.Reader) (*Message, error){
 
 	}
 
+	// return error if unmasked frame
+	if mask == nil {
+		return nil, UnmaskedFrameErr
+	}
+
 	return m, nil
 
 }
diff --git a/ws/message_test.go b/ws/message_test.go
new file mode 100644
index 0000000..5aba272
--- /dev/null
+++ b/ws/message_test.go
@@ -0,0 +1,80 @@
+package ws
+
+import (
+	"bytes"
+	"testing"
+)
+
+
+func TestSimpleMessageReading(t *testing.T) {
+
+	cases := []struct{
+		ReadBuffer []byte
+		Expected   Message
+		Err        error
+	}{
+		{ // FIN ; TEXT ; Unmasked -> error
+			[]byte{0x81,0x05,0x68,0x65,0x6c,0x6c,0x6f},
+			Message{},
+			UnmaskedFrameErr,
+		},
+		{ // FIN ; TEXT ; hello
+			[]byte{0x81,0x85,0x00,0x00,0x00,0x00,0x68,0x65,0x6c,0x6c,0x6f},
+			Message{ true, TEXT, 5, []byte("hello") },
+			nil,
+		},
+		{ // FIN ; BINARY ; hello
+			[]byte{0x82,0x85,0x00,0x00,0x00,0x00,0x68,0x65,0x6c,0x6c,0x6f},
+			Message{ true, BINARY, 5, []byte("hello") },
+			nil,
+		},
+		{ // FIN ; BINARY ; test unmasking
+			[]byte{0x82,0x88,0x01,0x02,0x03,0x04,0x10,0x20,0x30,0x40,0x50,0x60,0x70,0x80},
+			Message{ true, BINARY, 8, []byte{0x11,0x22,0x33,0x44,0x51,0x62,0x73,0x84} },
+			nil,
+		},
+		{ // FIN=0 ; TEXT ;
+			[]byte{0x01,0x82,0x00,0x00,0x00,0x00,0x01,0x02},
+			Message{ false, TEXT, 2, []byte{0x01,0x02} },
+			nil,
+		},
+	}
+
+	for _, tc := range cases{
+
+		reader := bytes.NewBuffer(tc.ReadBuffer)
+
+		got, err := readMessage(reader)
+
+		if err != tc.Err {
+			t.Errorf("Expected %v error, got %v", tc.Err, err)
+		}
+
+		// do not check message if error expected
+		if tc.Err != nil {
+			continue
+		}
+
+		// check FIN
+		if got.Final != tc.Expected.Final {
+			t.Errorf("Expected FIN=%t, got %t", tc.Expected.Final, got.Final)
+		}
+
+		// check OpCode
+		if got.Type != tc.Expected.Type {
+			t.Errorf("Expected TYPE=%x, got %x", tc.Expected.Type, got.Type)
+		}
+
+		// check Size
+		if got.Size != tc.Expected.Size {
+			t.Errorf("Expected SIZE=%d, got %d", tc.Expected.Size, got.Size)
+		}
+
+		// check Data
+		if string(got.Data) != string(tc.Expected.Data) {
+			t.Errorf("Expected Data='%s', got '%d'", tc.Expected.Data, got.Data)
+		}
+
+	}
+
+}
\ No newline at end of file