diff --git a/yarn-project/prover-client/src/light/lightweight_checkpoint_builder.test.ts b/yarn-project/prover-client/src/light/lightweight_checkpoint_builder.test.ts
index 0777f473ab69..218870d8a3af 100644
--- a/yarn-project/prover-client/src/light/lightweight_checkpoint_builder.test.ts
+++ b/yarn-project/prover-client/src/light/lightweight_checkpoint_builder.test.ts
@@ -328,5 +328,35 @@ describe('LightweightCheckpointBuilder', () => {
 
       await fork.close();
     });
+
+    it('adding a block with a mismatched block number fails with archive tree leaf index mismatch', async () => {
+      const checkpointNumber = CheckpointNumber(1);
+      const slotNumber = SlotNumber(15);
+
+      const constants = makeCheckpointConstants(slotNumber);
+      const l1ToL2Messages: Fr[] = [];
+      const previousCheckpointOutHashes: Fr[] = [];
+
+      const fork = await worldState.fork();
+
+      const checkpointBuilder = await LightweightCheckpointBuilder.startNewCheckpoint(
+        checkpointNumber,
+        constants,
+        l1ToL2Messages,
+        previousCheckpointOutHashes,
+        fork,
+      );
+
+      // Pass block number 5 when the archive tree expects block 1.
+      // After updateArchive, nextAvailableLeafIndex will be 2 but expectedNextLeafIndex will be 6.
+      const wrongBlockNumber = BlockNumber(5);
+      const globalVariables = makeGlobalVariables(wrongBlockNumber, slotNumber);
+
+      await expect(checkpointBuilder.addBlock(globalVariables, [], { insertTxsEffects: true })).rejects.toThrow(
+        /Archive tree next leaf index mismatch/,
+      );
+
+      await fork.close();
+    });
   });
 });
diff --git a/yarn-project/prover-client/src/light/lightweight_checkpoint_builder.ts b/yarn-project/prover-client/src/light/lightweight_checkpoint_builder.ts
index d8784c80cd39..1e72a75d7684 100644
--- a/yarn-project/prover-client/src/light/lightweight_checkpoint_builder.ts
+++ b/yarn-project/prover-client/src/light/lightweight_checkpoint_builder.ts
@@ -220,6 +220,13 @@ export class LightweightCheckpointBuilder {
     timings.updateArchive = msUpdateArchive;
     this.lastArchives.push(newArchive);
 
+    const expectedNextLeafIndex = Number(globalVariables.blockNumber) + 1;
+    if (newArchive.nextAvailableLeafIndex !== expectedNextLeafIndex) {
+      throw new Error(
+        `Archive tree next leaf index mismatch after building block ${globalVariables.blockNumber} (expected ${expectedNextLeafIndex} but got ${newArchive.nextAvailableLeafIndex})`,
+      );
+    }
+
     const indexWithinCheckpoint = IndexWithinCheckpoint(this.blocks.length);
     const block = new L2Block(newArchive, header, body, this.checkpointNumber, indexWithinCheckpoint);
     this.blocks.push(block);