diff --git a/packages/slate/src/interfaces/custom-types.ts b/packages/slate/src/interfaces/custom-types.ts
index c8999cd38..397112a05 100644
--- a/packages/slate/src/interfaces/custom-types.ts
+++ b/packages/slate/src/interfaces/custom-types.ts
@@ -2,10 +2,28 @@
  * Extendable Custom Types Interface
  */
 
+type ExtendableTypes =
+  | 'Editor'
+  | 'Element'
+  | 'Text'
+  | 'Selection'
+  | 'Range'
+  | 'Point'
+  | 'InsertNodeOperation'
+  | 'InsertTextOperation'
+  | 'MergeNodeOperation'
+  | 'MoveNodeOperation'
+  | 'RemoveNodeOperation'
+  | 'RemoveTextOperation'
+  | 'SetNodeOperation'
+  | 'SetSelectionOperation'
+  | 'SplitNodeOperation'
+
 export interface CustomTypes {
   [key: string]: unknown
 }
 
-export type ExtendedType<K extends string, B> = unknown extends CustomTypes[K]
-  ? B
-  : CustomTypes[K]
+export type ExtendedType<
+  K extends ExtendableTypes,
+  B
+> = unknown extends CustomTypes[K] ? B : CustomTypes[K]