diff --git a/packages/core/src/hydration/annotate.ts b/packages/core/src/hydration/annotate.ts
index bcb042d1ad7..a55f15c9ead 100644
--- a/packages/core/src/hydration/annotate.ts
+++ b/packages/core/src/hydration/annotate.ts
@@ -405,7 +405,6 @@ function serializeLContainer(
 
           // Add defer block into info context.deferBlocks
           const deferBlockInfo: SerializedDeferBlock = {
-            [DEFER_PARENT_BLOCK_ID]: parentDeferBlockId,
             [NUM_ROOT_NODES]: rootNodes.length,
             [DEFER_BLOCK_STATE]: lDetails[CURRENT_DEFER_BLOCK_STATE],
           };
@@ -415,6 +414,11 @@ function serializeLContainer(
             deferBlockInfo[DEFER_HYDRATE_TRIGGERS] = serializedTriggers;
           }
 
+          if (parentDeferBlockId !== null) {
+            // Serialize parent id only when it's present.
+            deferBlockInfo[DEFER_PARENT_BLOCK_ID] = parentDeferBlockId;
+          }
+
           context.deferBlocks.set(deferBlockId, deferBlockInfo);
 
           const node = unwrapRNode(lContainer);
diff --git a/packages/core/src/hydration/interfaces.ts b/packages/core/src/hydration/interfaces.ts
index 5808b179571..c66ee9dd3e5 100644
--- a/packages/core/src/hydration/interfaces.ts
+++ b/packages/core/src/hydration/interfaces.ts
@@ -158,7 +158,7 @@ export interface SerializedDeferBlock {
   /**
    * This contains the unique id of this defer block's parent, if it exists.
    */
-  [DEFER_PARENT_BLOCK_ID]: string | null;
+  [DEFER_PARENT_BLOCK_ID]?: string;
 
   /**
    * This field represents a status, based on the `DeferBlockState` enum.
diff --git a/packages/core/src/hydration/utils.ts b/packages/core/src/hydration/utils.ts
index eb9b549d359..49ee3b126e4 100644
--- a/packages/core/src/hydration/utils.ts
+++ b/packages/core/src/hydration/utils.ts
@@ -571,7 +571,7 @@ export function getParentBlockHydrationQueue(
   const deferBlockParents = transferState.get(NGH_DEFER_BLOCKS_KEY, {});
 
   let isTopMostDeferBlock = false;
-  let currentBlockId: string | null = deferBlockId;
+  let currentBlockId: string | undefined = deferBlockId;
   let parentBlockPromise: Promise<void> | null = null;
   const hydrationQueue: string[] = [];
 
diff --git a/packages/platform-server/test/incremental_hydration_spec.ts b/packages/platform-server/test/incremental_hydration_spec.ts
index 44e0c8b4665..69263d96389 100644
--- a/packages/platform-server/test/incremental_hydration_spec.ts
+++ b/packages/platform-server/test/incremental_hydration_spec.ts
@@ -222,7 +222,7 @@ describe('platform-server partial hydration integration', () => {
         const ssrContents = getAppContents(html);
 
         expect(ssrContents).toContain(
-          '"__nghDeferData__":{"d0":{"p":null,"r":1,"s":2},"d1":{"p":"d0","r":2,"s":2}}',
+          '"__nghDeferData__":{"d0":{"r":1,"s":2},"d1":{"r":2,"s":2,"p":"d0"}}',
         );
       });
 
@@ -285,9 +285,38 @@ describe('platform-server partial hydration integration', () => {
         const ssrContents = getAppContents(html);
 
         expect(ssrContents).toContain(
-          '"__nghDeferData__":{"d0":{"p":null,"r":1,"s":2},"d1":{"p":"d0","r":2,"s":2,"t":[2]}}',
+          '"__nghDeferData__":{"d0":{"r":1,"s":2},"d1":{"r":2,"s":2,"t":[2],"p":"d0"}}',
         );
       });
+
+      it('should not include parent id in serialized data for top-level `@defer` blocks', async () => {
+        @Component({
+          selector: 'app',
+          template: `
+            @defer (on viewport; hydrate on interaction) {
+              Hello world!
+            } @placeholder {
+              <span>Placeholder</span>
+            }
+        `,
+        })
+        class SimpleComponent {}
+
+        const appId = 'custom-app-id';
+        const providers = [{provide: APP_ID, useValue: appId}];
+        const hydrationFeatures = () => [withIncrementalHydration()];
+
+        const html = await ssr(SimpleComponent, {
+          envProviders: providers,
+          hydrationFeatures,
+        });
+
+        const ssrContents = getAppContents(html);
+
+        // Assert that the serialized data doesn't contain the "p" field,
+        // which contains parent id (which is not needed for top-level blocks).
+        expect(ssrContents).toContain('"__nghDeferData__":{"d0":{"r":1,"s":2}}}');
+      });
     });
 
     describe('basic hydration behavior', () => {
@@ -347,7 +376,7 @@ describe('platform-server partial hydration integration', () => {
         expect(ssrContents).toContain('<p jsaction="click:;keydown:;" ngb="d1');
         // There is an extra annotation in the TransferState data.
         expect(ssrContents).toContain(
-          '"__nghDeferData__":{"d0":{"p":null,"r":1,"s":2},"d1":{"p":"d0","r":1,"s":2}}',
+          '"__nghDeferData__":{"d0":{"r":1,"s":2},"d1":{"r":1,"s":2,"p":"d0"}}',
         );
         // Outer defer block is rendered.
         expect(ssrContents).toContain('Main defer block rendered');
@@ -460,7 +489,7 @@ describe('platform-server partial hydration integration', () => {
         expect(ssrContents).toContain('<p jsaction="click:;keydown:;" ngb="d1');
         // There is an extra annotation in the TransferState data.
         expect(ssrContents).toContain(
-          '"__nghDeferData__":{"d0":{"p":null,"r":1,"s":2},"d1":{"p":"d0","r":1,"s":2}}',
+          '"__nghDeferData__":{"d0":{"r":1,"s":2},"d1":{"r":1,"s":2,"p":"d0"}}',
         );
         // Outer defer block is rendered.
         expect(ssrContents).toContain('Main defer block rendered');
@@ -569,7 +598,7 @@ describe('platform-server partial hydration integration', () => {
         // <p> is inside a nested defer block -> different namespace.
         // expect(ssrContents).toContain('<p jsaction="click:;" ngb="d1');
         // There is an extra annotation in the TransferState data.
-        expect(ssrContents).toContain('"__nghDeferData__":{"d0":{"p":null,"r":1,"s":2}}');
+        expect(ssrContents).toContain('"__nghDeferData__":{"d0":{"r":1,"s":2}}');
         // Outer defer block is rendered.
         expect(ssrContents).toContain('Main defer block rendered');
         // Inner defer block should only display placeholder.