diff --git a/build/vega-lite-schema.json b/build/vega-lite-schema.json
index 9f04f10c9e..32ba92d003 100644
--- a/build/vega-lite-schema.json
+++ b/build/vega-lite-schema.json
@@ -976,7 +976,7 @@
       "additionalProperties": false,
       "properties": {
         "argmax": {
-          "type": "string"
+          "$ref": "#/definitions/FieldName"
         }
       },
       "required": [
@@ -988,7 +988,7 @@
       "additionalProperties": false,
       "properties": {
         "argmin": {
-          "type": "string"
+          "$ref": "#/definitions/FieldName"
         }
       },
       "required": [
diff --git a/src/aggregate.ts b/src/aggregate.ts
index dd10cec54a..1e0c5f8502 100644
--- a/src/aggregate.ts
+++ b/src/aggregate.ts
@@ -1,5 +1,6 @@
 import {AggregateOp} from 'vega';
 import {isString, toSet} from 'vega-util';
+import {FieldName} from './channeldef';
 import {contains, Flag, keys} from './util';
 
 const AGGREGATE_OP_INDEX: Flag<AggregateOp> = {
@@ -35,11 +36,11 @@ export const MULTIDOMAIN_SORT_OP_INDEX = {
 };
 
 export interface ArgminDef {
-  argmin: string;
+  argmin: FieldName;
 }
 
 export interface ArgmaxDef {
-  argmax: string;
+  argmax: FieldName;
 }
 
 export type NonArgAggregateOp = Exclude<AggregateOp, 'argmin' | 'argmax'>;