diff --git a/CHANGELOG.md b/CHANGELOG.md
index e2c30025..694b07d4 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -15,6 +15,7 @@ If you are upgrading from v0.16.x, replace the binary (or run `docker pull`). If
 - OIDC: `ECDSA` private key support for `SEC1` format.
 - PostgreSQL: Include error chain in error messages.
 - Prometheus: event counters are exported with incorrect metric names.
+- Registry: Changing the type of an existing account from `user` to `group` panics.
 
 ## [0.16.11] - 2026-06-25
 
diff --git a/crates/jmap/src/registry/mapping/principal.rs b/crates/jmap/src/registry/mapping/principal.rs
index d8dc5765..0594ed4e 100644
--- a/crates/jmap/src/registry/mapping/principal.rs
+++ b/crates/jmap/src/registry/mapping/principal.rs
@@ -245,7 +245,14 @@ pub(crate) async fn validate_account(
 
             true
         }
-        _ => unreachable!(),
+        (Account::User(_), AccountUpdate::Update(Account::Group(_)))
+        | (Account::Group(_), AccountUpdate::Update(Account::User(_))) => {
+            return Ok(Err(SetError::invalid_properties()
+                .with_property(Property::Type)
+                .with_description(
+                    "Cannot change the type of an existing account.",
+                )));
+        }
     };
 
     let mut result = if validate_permissions {