diff --git a/src/TreeDataGrid.tsx b/src/TreeDataGrid.tsx
index 4b855b26c6..7aea6211aa 100644
--- a/src/TreeDataGrid.tsx
+++ b/src/TreeDataGrid.tsx
@@ -36,7 +36,7 @@ export interface TreeDataGridProps<R, SR = unknown, K extends Key = Key>
   ) => Record<string, readonly NoInfer<R>[]>;
   expandedGroupIds: ReadonlySet<unknown>;
   onExpandedGroupIdsChange: (expandedGroupIds: Set<unknown>) => void;
-  generateGroupId: ((groupKey: string, parentId?: string) => string) | undefined;
+  generateGroupId?: (groupKey: string, parentId?: string) => string;
 }
 
 type GroupByDictionary<TRow> = Record<
diff --git a/test/TreeDataGrid.test.tsx b/test/TreeDataGrid.test.tsx
index 5e7630e9fe..0f55399445 100644
--- a/test/TreeDataGrid.test.tsx
+++ b/test/TreeDataGrid.test.tsx
@@ -127,7 +127,7 @@ function TestGrid({ groupBy, generateGroupId }: TestGridProps) {
       onExpandedGroupIdsChange={setExpandedGroupIds}
       onRowsChange={setRows}
       onPaste={onPaste}
-      generateGroupId={generateGroupId}
+      {...(generateGroupId && { generateGroupId })}
     />
   );
 }