diff --git a/packages/core/src/hydration/annotate.ts b/packages/core/src/hydration/annotate.ts
index 64c75efe4c4e99..1402e8db8c1e64 100644
--- a/packages/core/src/hydration/annotate.ts
+++ b/packages/core/src/hydration/annotate.ts
@@ -405,12 +405,16 @@ 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],
             [DEFER_HYDRATE_TRIGGERS]: serializeHydrateTriggers(tDetails.hydrateTriggers),
           };
 
+          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 929cdc3e3df6cd..7ac11311df3393 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 14692c41c2f856..31c036a0cd7577 100644
--- a/packages/core/src/hydration/utils.ts
+++ b/packages/core/src/hydration/utils.ts
@@ -566,7 +566,7 @@ 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 e46dd96902e589..c706fd48e2a2fa 100644
--- a/packages/platform-server/test/incremental_hydration_spec.ts
+++ b/packages/platform-server/test/incremental_hydration_spec.ts
@@ -206,7 +206,7 @@ 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,"t":[]},"d1":{"p":"d0","r":1,"s":2,"t":[]}}',
+        '"__nghDeferData__":{"d0":{"r":1,"s":2,"t":[]},"d1":{"r":1,"s":2,"t":[],"p":"d0"}}',
       );
       // Outer defer block is rendered.
       expect(ssrContents).toContain('Main defer block rendered');
@@ -319,7 +319,7 @@ 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,"t":[]},"d1":{"p":"d0","r":1,"s":2,"t":[]}}',
+        '"__nghDeferData__":{"d0":{"r":1,"s":2,"t":[]},"d1":{"r":1,"s":2,"t":[],"p":"d0"}}',
       );
       // Outer defer block is rendered.
       expect(ssrContents).toContain('Main defer block rendered');
@@ -429,7 +429,7 @@ describe('platform-server partial hydration integration', () => {
       // 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,"t":[]},"d1":{"p":"d0","r":1,"s":0,"t":null}}',
+        '"__nghDeferData__":{"d0":{"r":1,"s":2,"t":[]},"d1":{"r":1,"s":0,"t":null,"p":"d0"}}',
       );
       // Outer defer block is rendered.
       expect(ssrContents).toContain('Main defer block rendered');