diff --git a/src/pymp4/parser.py b/src/pymp4/parser.py
index 0d7c943..1dd0143 100644
--- a/src/pymp4/parser.py
+++ b/src/pymp4/parser.py
@@ -298,7 +298,7 @@
         "enca": MP4ASampleEntryBox,
         "avc1": AVC1SampleEntryBox,
         "encv": AVC1SampleEntryBox,
-        "wvtt": Struct("children" / LazyBound(lambda ctx: GreedyRange(Box)))
+        "wvtt": Struct("children" / LazyBound(lambda: GreedyRange(Box)))
     }, GreedyBytes)
 ), includelength=True)
 
@@ -552,7 +552,7 @@
 TrackEncryptionBox = Struct(
     "version" / Default(OneOf(Int8ub, (0, 1)), 0),
     "flags" / Default(Int24ub, 0),
-    "_reserved" / Const(Int8ub, 0),
+    "_reserved" / Const(0, Int8ub),
     "default_byte_blocks" / Default(IfThenElse(
         this.version > 0,
         BitStruct(
@@ -561,7 +561,7 @@
             # count of unencrypted blocks in the protection pattern
             "skip" / Nibble
         ),
-        Const(Int8ub, 0)
+        Const(0, Int8ub)
     ), 0),
     "is_encrypted" / OneOf(Int8ub, (0, 1)),
     "iv_size" / OneOf(Int8ub, (0, 8, 16)),
@@ -620,31 +620,26 @@
 # WebVTT boxes
 
 CueIDBox = Struct(
-    "type" / Const(b"iden"),
     "cue_id" / GreedyString("utf8")
 )
 
 CueSettingsBox = Struct(
-    "type" / Const(b"sttg"),
     "settings" / GreedyString("utf8")
 )
 
 CuePayloadBox = Struct(
-    "type" / Const(b"payl"),
     "cue_text" / GreedyString("utf8")
 )
 
 WebVTTConfigurationBox = Struct(
-    "type" / Const(b"vttC"),
     "config" / GreedyString("utf8")
 )
 
 WebVTTSourceLabelBox = Struct(
-    "type" / Const(b"vlab"),
     "label" / GreedyString("utf8")
 )
 
-ContainerBoxLazy = LazyBound(lambda ctx: ContainerBox)
+ContainerBoxLazy = LazyBound(lambda: ContainerBox)
 
 
 class TellMinusSizeOf(Subconstruct):
diff --git a/tests/test_webvtt_boxes.py b/tests/test_webvtt_boxes.py
index 84f7db3..e322c2c 100644
--- a/tests/test_webvtt_boxes.py
+++ b/tests/test_webvtt_boxes.py
@@ -12,79 +12,109 @@ class BoxTests(unittest.TestCase):
     def test_iden_parse(self):
         self.assertEqual(
             Box.parse(b'\x00\x00\x00\x27iden2 - this is the second subtitle'),
-            Container(offset=0)
-            (type=b"iden")
-            (cue_id="2 - this is the second subtitle")
-            (end=39)
+            Container(
+                offset=0,
+                type="iden",
+                data=Container(
+                    cue_id="2 - this is the second subtitle"
+                ),
+                end=39
+            )
         )
 
     def test_iden_build(self):
         self.assertEqual(
             Box.build(dict(
-                type=b"iden",
-                cue_id="1 - first subtitle")),
+                type="iden",
+                data=dict(
+                    cue_id="1 - first subtitle"
+                ))),
             b'\x00\x00\x00\x1aiden1 - first subtitle')
 
     def test_sttg_parse(self):
         self.assertEqual(
             Box.parse(b'\x00\x00\x003sttgline:10% position:50% size:48% align:center'),
-            Container(offset=0)
-            (type=b"sttg")
-            (settings="line:10% position:50% size:48% align:center")
-            (end=51)
+            Container(
+                offset=0,
+                type="sttg",
+                data=Container(
+                    settings="line:10% position:50% size:48% align:center"
+                ),
+                end=51
+            )
         )
 
     def test_sttg_build(self):
         self.assertEqual(
             Box.build(dict(
-                type=b"sttg",
-                settings="line:75% position:20% size:2em align:right")),
+                type="sttg",
+                data=dict(
+                    settings="line:75% position:20% size:2em align:right"
+                ))),
             b'\x00\x00\x002sttgline:75% position:20% size:2em align:right')
 
     def test_payl_parse(self):
         self.assertEqual(
             Box.parse(b'\x00\x00\x00\x13payl[chuckling]'),
-            Container(offset=0)
-            (type=b"payl")
-            (cue_text="[chuckling]")
-            (end=19)
+            Container(
+                offset=0,
+                type="payl",
+                data=Container(
+                    cue_text="[chuckling]"
+                ),
+                end=19
+            )
         )
 
     def test_payl_build(self):
         self.assertEqual(
             Box.build(dict(
-                type=b"payl",
-                cue_text="I have a bad feeling about- [boom]")),
+                type="payl",
+                data=dict(
+                    cue_text="I have a bad feeling about- [boom]"
+                ))),
             b'\x00\x00\x00*paylI have a bad feeling about- [boom]')
 
     def test_vttC_parse(self):
         self.assertEqual(
             Box.parse(b'\x00\x00\x00\x0evttCWEBVTT'),
-            Container(offset=0)
-            (type=b"vttC")
-            (config="WEBVTT")
-            (end=14)
+            Container(
+                offset=0,
+                type="vttC",
+                data=Container(
+                    config="WEBVTT"
+                ),
+                end=14
+            )
         )
 
     def test_vttC_build(self):
         self.assertEqual(
             Box.build(dict(
-                type=b"vttC",
-                config="WEBVTT with a text header\n\nSTYLE\n::cue {\ncolor: red;\n}")),
+                type="vttC",
+                data=dict(
+                    config="WEBVTT with a text header\n\nSTYLE\n::cue {\ncolor: red;\n}"
+                ))),
             b'\x00\x00\x00>vttCWEBVTT with a text header\n\nSTYLE\n::cue {\ncolor: red;\n}')
 
     def test_vlab_parse(self):
         self.assertEqual(
             Box.parse(b'\x00\x00\x00\x14vlabsource_label'),
-            Container(offset=0)
-            (type=b"vlab")
-            (label="source_label")
-            (end=20)
+            Container(
+                offset=0,
+                type="vlab",
+                data=Container(
+                    label="source_label"
+                ),
+                end=20
+            )
         )
 
     def test_vlab_build(self):
         self.assertEqual(
             Box.build(dict(
-                type=b"vlab",
-                label="1234 \n test_label \n\n")),
+                type="vlab",
+                data=dict(
+                    label="1234 \n test_label \n\n"
+                ))),
             b'\x00\x00\x00\x1cvlab1234 \n test_label \n\n')