diff --git a/build/vega-lite-schema.json b/build/vega-lite-schema.json
index ecddd29bdd..9f04f10c9e 100644
--- a/build/vega-lite-schema.json
+++ b/build/vega-lite-schema.json
@@ -18648,8 +18648,8 @@
           "type": "number"
         },
         "op": {
-          "description": "The aggregation operation to apply to grouped `value` field values. __Default value:__ `sum`",
-          "type": "string"
+          "$ref": "#/definitions/AggregateOp",
+          "description": "The aggregation operation to apply to grouped `value` field values. __Default value:__ `sum`"
         },
         "pivot": {
           "$ref": "#/definitions/FieldName",
diff --git a/src/transform.ts b/src/transform.ts
index 81a720f792..2deb5ecdf4 100644
--- a/src/transform.ts
+++ b/src/transform.ts
@@ -416,7 +416,7 @@ export interface PivotTransform {
    * The aggregation operation to apply to grouped `value` field values.
    * __Default value:__ `sum`
    */
-  op?: string;
+  op?: AggregateOp;
 }
 
 export function isPivot(t: Transform): t is PivotTransform {