diff --git a/CHANGELOG.md b/CHANGELOG.md
index 56b79483..c4feb380 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -8,6 +8,10 @@ Changelog follow https://keepachangelog.com/ format.
 
 ## [Unreleased]
 
+*   `edc`:
+    *   Changed: Raise an error if `allow_unfrozen=` is used on a non-frozen
+        dataclass
+
 ## [1.5.0] - 2023-09-19
 
 *   `ecolab`:
diff --git a/etils/edc/frozen_utils.py b/etils/edc/frozen_utils.py
index 5ee645dd..a6338c6b 100644
--- a/etils/edc/frozen_utils.py
+++ b/etils/edc/frozen_utils.py
@@ -30,6 +30,11 @@
 
 def add_unfrozen(cls: _Cls) -> _Cls:
   """Add the `frozen`, `unfrozen` methods."""
+  if not cls.__dataclass_params__.frozen:
+    raise ValueError(
+        'allow_unfrozen require the dataclass to be defined with'
+        f' `frozen=True`. For {cls}'
+    )
   cls_frozen = getattr(cls, 'frozen', None)
   cls_unfrozen = getattr(cls, 'has_unfrozen', None)