diff --git a/packages/core/src/model-definition.ts b/packages/core/src/model-definition.ts
index 9ba1803607d4..bd2efdb2d86d 100644
--- a/packages/core/src/model-definition.ts
+++ b/packages/core/src/model-definition.ts
@@ -201,7 +201,7 @@ export class ModelDefinition<M extends Model = Model> {
 
     // TODO: deep freeze this.options
     // caution: mergeModelOptions mutates its first input
-    this.options = mergeModelOptions(
+    this.options = mergeModelOptions<M>(
       // default options
       {
         noPrimaryKey: false,
@@ -959,11 +959,11 @@ function banReferenceModel<T>(reference: T): T {
  * @param options
  * @param overrideOnConflict
  */
-export function mergeModelOptions(
-  existingModelOptions: ModelOptions,
-  options: ModelOptions,
+export function mergeModelOptions<M extends Model>(
+  existingModelOptions: ModelOptions<M>,
+  options: ModelOptions<M>,
   overrideOnConflict: boolean,
-): ModelOptions {
+): ModelOptions<M> {
   // merge-able: scopes, indexes
   for (const [optionName, optionValue] of Object.entries(options) as Array<
     [keyof ModelOptions, any]
diff --git a/packages/core/src/model.d.ts b/packages/core/src/model.d.ts
index 00986abd2df5..13aee05d7796 100644
--- a/packages/core/src/model.d.ts
+++ b/packages/core/src/model.d.ts
@@ -2115,7 +2115,7 @@ export interface ModelOptions<M extends Model = Model> {
         /**
          * Custom validation functions run on all instances of the model.
          */
-        [name: string]: (value: unknown) => boolean;
+        [name: string]: ((this: M) => void) | ((this: M, callback: (err: unknown) => void) => void);
       }
     | undefined;