diff --git a/netlib/websockets.py b/netlib/websockets.py
index 0cd4dba15..1e9c96cca 100644
--- a/netlib/websockets.py
+++ b/netlib/websockets.py
@@ -22,11 +22,17 @@ from . import utils, odict
 # RFC6455
 websockets_magic = '258EAFA5-E914-47DA-95CA-C5AB0DC85B11'
 VERSION = "13"
+MAX_16_BIT_INT = (1 << 16)
+MAX_64_BIT_INT = (1 << 64)
 
 
-class CONST(object):
-    MAX_16_BIT_INT = (1 << 16)
-    MAX_64_BIT_INT = (1 << 64)
+class OPCODE:
+    CONTINUE = 0x00
+    TEXT = 0x01
+    BINARY = 0x02
+    CLOSE = 0x08
+    PING = 0x09
+    PONG = 0x0a
 
 
 class Frame(object):
@@ -101,7 +107,7 @@ class Frame(object):
 
         return cls(
             fin = 1, # final frame
-            opcode = 1, # text
+            opcode = OPCODE.TEXT, # text
             mask_bit = mask_bit,
             payload_length_code = length_code,
             payload = payload,
@@ -115,28 +121,27 @@ class Frame(object):
             Validate websocket frame invariants, call at anytime to ensure the
             Frame has not been corrupted.
         """
-        try:
-            assert 0 <= self.fin <= 1
-            assert 0 <= self.rsv1 <= 1
-            assert 0 <= self.rsv2 <= 1
-            assert 0 <= self.rsv3 <= 1
-            assert 1 <= self.opcode <= 4
-            assert 0 <= self.mask_bit <= 1
-            assert 1 <= self.payload_length_code <= 127
-
-            if self.mask_bit == 1:
-                assert 1 <= len(self.masking_key) <= 4
-            else:
-                assert self.masking_key is None
-
-            assert self.actual_payload_length == len(self.payload)
-
-            if self.payload is not None and self.masking_key is not None:
-                assert apply_mask(self.payload, self.masking_key) == self.decoded_payload
-
-            return True
-        except AssertionError:
+        constraints = [
+            0 <= self.fin <= 1,
+            0 <= self.rsv1 <= 1,
+            0 <= self.rsv2 <= 1,
+            0 <= self.rsv3 <= 1,
+            1 <= self.opcode <= 4,
+            0 <= self.mask_bit <= 1,
+            1 <= self.payload_length_code <= 127,
+            self.actual_payload_length == len(self.payload)
+        ]
+        if not all(constraints):
             return False
+        elif self.mask_bit == 1 and not 1 <= len(self.masking_key) <= 4:
+            return False
+        elif self.mask_bit == 0 and self.masking_key is not None:
+            return False
+        elif self.payload and self.masking_key:
+            decoded = apply_mask(self.payload, self.masking_key)
+            if decoded != self.decoded_payload:
+                return False
+        return True
 
     def human_readable(self): # pragma: nocover
         return "\n".join([
@@ -192,11 +197,11 @@ class Frame(object):
 
         if self.actual_payload_length < 126:
             pass
-        elif self.actual_payload_length < CONST.MAX_16_BIT_INT:
+        elif self.actual_payload_length < MAX_16_BIT_INT:
             # '!H' pack as 16 bit unsigned short
             # add 2 byte extended payload length
             b += struct.pack('!H', self.actual_payload_length)
-        elif self.actual_payload_length < CONST.MAX_64_BIT_INT:
+        elif self.actual_payload_length < MAX_64_BIT_INT:
             # '!Q' = pack as 64 bit unsigned long long
             # add 8 bytes extended payload length
             b += struct.pack('!Q', self.actual_payload_length)
@@ -212,15 +217,15 @@ class Frame(object):
         writer.flush()
 
     @classmethod
-    def from_file(cls, reader):
+    def from_file(cls, fp):
         """
           read a websockets frame sent by a server or client
 
-          reader is a "file like" object that could be backed by a network
+          fp is a "file like" object that could be backed by a network
           stream or a disk or an in memory stream reader
         """
-        first_byte = utils.bytes_to_int(reader.read(1))
-        second_byte = utils.bytes_to_int(reader.read(1))
+        first_byte = utils.bytes_to_int(fp.read(1))
+        second_byte = utils.bytes_to_int(fp.read(1))
 
         # grab the left most bit
         fin = first_byte >> 7
@@ -237,18 +242,18 @@ class Frame(object):
             actual_payload_length = payload_length
 
         elif payload_length == 126:
-            actual_payload_length = utils.bytes_to_int(reader.read(2))
+            actual_payload_length = utils.bytes_to_int(fp.read(2))
 
         elif payload_length == 127:
-            actual_payload_length = utils.bytes_to_int(reader.read(8))
+            actual_payload_length = utils.bytes_to_int(fp.read(8))
 
         # masking key only present if mask bit set
         if mask_bit == 1:
-            masking_key = reader.read(4)
+            masking_key = fp.read(4)
         else:
             masking_key = None
 
-        payload = reader.read(actual_payload_length)
+        payload = fp.read(actual_payload_length)
 
         if mask_bit == 1:
             decoded_payload = apply_mask(payload, masking_key)
diff --git a/test/test_websockets.py b/test/test_websockets.py
index 3fc67dfee..9e205e701 100644
--- a/test/test_websockets.py
+++ b/test/test_websockets.py
@@ -108,6 +108,33 @@ class TestWebSockets(test.ServerTestBase):
         server_frame = websockets.Frame.default(msg, from_client = False)
         assert server_frame.is_valid()
 
+    def test_is_valid(self):
+        def f():
+            return websockets.Frame.default(self.random_bytes(10), True)
+
+        frame = f()
+        assert frame.is_valid()
+
+        frame = f()
+        frame.fin = 2
+        assert not frame.is_valid()
+
+        frame = f()
+        frame.mask_bit = 1
+        frame.masking_key = "foobbarboo"
+        assert not frame.is_valid()
+
+        frame = f()
+        frame.mask_bit = 0
+        frame.masking_key = "foob"
+        assert not frame.is_valid()
+
+        frame = f()
+        frame.masking_key = "foob"
+        frame.decoded_payload = "xxxx"
+        assert not frame.is_valid()
+
+
     def test_serialization_bijection(self):
         """
           Ensure that various frame types can be serialized/deserialized back