diff --git a/.pre-commit-config.yaml b/.pre-commit-config.yaml
index 7b3fc9324fae08fceaf805353421a115f1d02397..1699979901b6d1c765b8cbe11d6bec3155541920 100644
--- a/.pre-commit-config.yaml
+++ b/.pre-commit-config.yaml
@@ -97,6 +97,6 @@ repos:
     pass_filenames: false
     additional_dependencies:
     - anyio ~=3.1
-    - kilter.protocol ~=0.4.0
+    - kilter.protocol ~=0.5.0
     - sphinx
     - trio-typing
diff --git a/kilter/service/session.py b/kilter/service/session.py
index 090d318593b648b9e6c301763f2fc38453ccc9d0..8c7fe502b668dd7c1f3ffd776ec4fef6706ac573 100644
--- a/kilter/service/session.py
+++ b/kilter/service/session.py
@@ -375,6 +375,7 @@ class HeadersAccessor(AsyncContextManager["HeaderIterator"]):
 		while self.session.phase <= Phase.HEADERS:
 			match (await self.session.broadcast.receive()):
 				case Header() as header:
+					header.freeze()
 					self._table.append(header)
 					seen.add(id(header))
 					try:
@@ -403,6 +404,7 @@ class HeadersAccessor(AsyncContextManager["HeaderIterator"]):
 		while self.session.phase <= Phase.HEADERS:
 			match (await self.session.broadcast.receive()):
 				case Header() as header:
+					header.freeze()
 					self._table.append(header)
 				case EndOfHeaders():
 					return
diff --git a/pyproject.toml b/pyproject.toml
index 2de7a08dca60913a3e6c49dd5a59dbec87d30786..20c81fd9b54497d0761fae0c153e7dfdcea96668 100644
--- a/pyproject.toml
+++ b/pyproject.toml
@@ -15,7 +15,7 @@ requires-python = "~=3.10"
 dependencies = [
 	"anyio ~=3.0",
 	"async-generator ~=1.2",
-	"kilter.protocol ~=0.4.0",
+	"kilter.protocol ~=0.5.0",
 	"typing-extensions ~=4.0",
 ]
 classifiers = [