immutable-js
diff --git a/‎__tests__/regression.ts
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+9Lines changed: 9 additions & 0 deletions b/‎__tests__/regression.ts
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+9Lines changed: 9 additions & 0 deletions
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+import { describe, expect, it } from '@jest/globals';
+import { List } from 'immutable';
+
+describe('undefined groupBy', () => {
+  it('should not throw an error when groupBy is called with undefined', () => {
+    const list = List([1, 2, 3]);
+    expect(() => list.groupBy(() => undefined)).not.toThrow();
+  });
+});