diff --git a/.changeset/clear-impalas-stare.md b/.changeset/clear-impalas-stare.md
new file mode 100644
index 0000000..5217f16
--- /dev/null
+++ b/.changeset/clear-impalas-stare.md
@@ -0,0 +1,5 @@
+---
+'@electric-sql/d2mini': patch
+---
+
+make aggregates explicitly optional on a groupBy
diff --git a/packages/d2mini/src/operators/groupBy.ts b/packages/d2mini/src/operators/groupBy.ts
index b36bbbe..8904106 100644
--- a/packages/d2mini/src/operators/groupBy.ts
+++ b/packages/d2mini/src/operators/groupBy.ts
@@ -40,7 +40,7 @@ export function groupBy<
   T,
   K extends GroupKey,
   A extends Record<string, AggregateFunction<T, any, any>>,
->(keyExtractor: (data: T) => K, aggregates: A) {
+>(keyExtractor: (data: T) => K, aggregates: A = {} as A) {
   type ResultType = K & AggregatesReturnType<T, A>
 
   const basicAggregates = Object.fromEntries(
diff --git a/packages/d2mini/tests/operators/groupBy.test.ts b/packages/d2mini/tests/operators/groupBy.test.ts
index d342ee9..c147620 100644
--- a/packages/d2mini/tests/operators/groupBy.test.ts
+++ b/packages/d2mini/tests/operators/groupBy.test.ts
@@ -15,6 +15,62 @@ import { output } from '../../src/operators/index.js'
 
 describe('Operators', () => {
   describe('GroupBy operation', () => {
+    test('with no aggregate', () => {
+      const graph = new D2()
+      const input = graph.newInput<{
+        category: string
+        amount: number
+      }>()
+      let latestMessage: any = null
+
+      input.pipe(
+        groupBy((data) => ({ category: data.category })),
+        output((message) => {
+          latestMessage = message
+        }),
+      )
+
+      graph.finalize()
+
+      // Initial data
+      input.sendData(
+        new MultiSet([
+          [{ category: 'A', amount: 10 }, 1],
+          [{ category: 'A', amount: 20 }, 1],
+          [{ category: 'B', amount: 30 }, 1],
+        ]),
+      )
+      graph.run()
+
+      // Verify we have the latest message
+      expect(latestMessage).not.toBeNull()
+
+      const result = latestMessage.getInner()
+
+      const expectedResult = [
+        [
+          [
+            `{"category":"A"}`,
+            {
+              category: 'A',
+            },
+          ],
+          1,
+        ],
+        [
+          [
+            `{"category":"B"}`,
+            {
+              category: 'B',
+            },
+          ],
+          1,
+        ],
+      ]
+
+      expect(result).toEqual(expectedResult)
+    })
+
     test('with single sum aggregate', () => {
       const graph = new D2()
       const input = graph.newInput<{