diff --git a/crates/ui-egui/src/panel_docking.rs b/crates/ui-egui/src/panel_docking.rs
index 8d811230a..7e1255dba 100644
--- a/crates/ui-egui/src/panel_docking.rs
+++ b/crates/ui-egui/src/panel_docking.rs
@@ -491,6 +491,11 @@ fn shared_command(app: &mut DesignApp, id: &str, params: &Value) -> Result<Value
         DockCommand::Window { .. } => return Err("DesignCraft does not move panels to their own windows".into()),
         _ => {}
     }
+    // A zone is a side of `anchor`. DesignCraft's docks are columns beside the canvas, so a dock at
+    // a window edge is a `side`, not a frame on the outer edge (craft-ui's `Placement::Edge`).
+    if let DockCommand::Dock { anchor: None, side: None, zone: Some(_), .. } = &command {
+        return Err("zone needs an anchor: dock beside a panel with anchor, or in a new dock column with side (Start or End)".into());
+    }
     let rtl = crate::i18n::is_rtl(&app.ui.language);
     let space = float_space(app);
     let mut txn = Transaction::begin(app)?;
diff --git a/crates/ui-egui/src/panel_docking_tests.rs b/crates/ui-egui/src/panel_docking_tests.rs
index 4638c8320..66416ba6d 100644
--- a/crates/ui-egui/src/panel_docking_tests.rs
+++ b/crates/ui-egui/src/panel_docking_tests.rs
@@ -997,10 +997,15 @@ fn shared_panel_commands_float_dock_and_reject_atomically() {
         ("window.panel.window", json!({"panel": "pages"})),
         ("window.panel.move", json!({"panel": "stroke", "anchor": "pages", "zone": "left", "before": "pages"})),
         ("window.panel.dock", json!({"panel": "stroke", "side": "Middle"})),
+        ("window.panel.dock", json!({"panel": "stroke", "zone": "left"})),
+        ("window.dockPanel", json!({"panel": "properties", "zone": "bottom"})),
     ] {
         assert!(app.run(id, params).is_err(), "{id}");
         assert_eq!((app.ui.docking.clone(), app.ui.docking_hidden.clone()), before, "{id}");
     }
+    // A zone is a side of an anchor; the error names the two ways to say where.
+    let error = app.run("window.panel.dock", json!({"panel": "stroke", "zone": "right"})).unwrap_err();
+    assert!(error.contains("anchor") && error.contains("side"), "{error}");
     // Every shared id is registered for the command list and MCP.
     for id in craft_ui::docking::DockCommand::<String>::IDS {
         assert!(crate::menus::UI_COMMANDS.iter().any(|c| c.0 == *id), "{id} is not registered");
diff --git a/docs/control-protocol.md b/docs/control-protocol.md
index 7c1f946cd..0d36c021a 100644
--- a/docs/control-protocol.md
+++ b/docs/control-protocol.md
@@ -116,7 +116,9 @@ read as `anchor`.
   rect unless the params change it.
 - `window.panel.dock {panel, anchor?, zone?, before?, group?, column?, side?}` (and
   `window.dockPanel`) docks beside `anchor`, or in a new column of the dock at `side`
-  (`Start` or `End`; a new dock when there is none). Without either, a panel that floated from
+  (`Start` or `End`; a new dock when there is none). A panel that already leads that dock's
+  last column stays where it is. `zone` places the panel beside `anchor`, so a `zone` without
+  `anchor` or `side` is an error. Without `anchor` and `side`, a panel that floated from
   the dock goes back to that place, and another floating panel joins the first dock group. For
   the float-only Tabs panel (`tabs`), which has its own window outside the layout, float opens
   that window and dock closes it.