diff --git a/packages/upload-client/src/sharding.js b/packages/upload-client/src/sharding.js
index 39848f566..daf8a0711 100644
--- a/packages/upload-client/src/sharding.js
+++ b/packages/upload-client/src/sharding.js
@@ -105,7 +105,7 @@ export class ShardingStream extends TransformStream {
           overflowCurrentLength = 0
           /** @type {Map<import('./types.js').SliceDigest, import('./types.js').Position>} */
           const overflowSlices = new DigestMap()
-          for (const block of blocks) {
+          for (const block of overflowBlocks) {
             const overflowBlockHeaderLength = blockHeaderEncodingLength(block)
             overflowSlices.set(block.cid.multihash, [
               headerLength + overflowCurrentLength + overflowBlockHeaderLength,
diff --git a/packages/upload-client/test/sharding.test.js b/packages/upload-client/test/sharding.test.js
index 79a3ef2ab..6c1444c32 100644
--- a/packages/upload-client/test/sharding.test.js
+++ b/packages/upload-client/test/sharding.test.js
@@ -78,24 +78,22 @@ describe('ShardingStream', () => {
       await randomBlock(32), // encoded block length = 70
     ]
 
-    /** @type {import('../src/types.js').CARFile[]} */
+    /** @type {import('../src/types.js').IndexedCARFile[]} */
     const shards = []
+    let i = 0
     await new ReadableStream({
       pull(controller) {
-        const block = blocks.shift()
+        const block = blocks[i]
         if (!block) return controller.close()
         controller.enqueue(block)
+        i++
       },
     })
-      // shard with no roots = encoded block (166) + CAR header (18) = 183
-      // shard with no roots = encoded block (102) + CAR header (18) = 120
-      // shard with 1 root = encoded block (70) + CAR header (18) = 88
-      // shard with 1 root = encoded block (70) + CAR header (59) = 155
-      // i.e. shard size of 208 (120 + 88) should allow us 1 shard with 0 roots
-      // and then 1 shard with 2 blocks that, when encoded as a CAR with 1 root
-      // will actually exceed the shard size. It must then be refactored into
-      // 2 shards.
-      .pipeThrough(new ShardingStream({ shardSize: 208 }))
+      // 166 + 102 + 70 + 18 (0 root CAR header) = 356
+      // 166 + 102 + 70 + 59 (1 root CAR header) = 397
+      // Choose 360 as shard size so when CAR header with a root is added, the
+      // 3rd block is moved into a new shard.
+      .pipeThrough(new ShardingStream({ shardSize: 360 }))
       .pipeTo(
         new WritableStream({
           write: (s) => {
@@ -104,7 +102,39 @@ describe('ShardingStream', () => {
         })
       )
 
-    assert.equal(shards.length, 3)
+    assert.equal(shards.length, 2)
+
+    const shard0Bytes = new Uint8Array(await shards[0].arrayBuffer())
+    const shard1Bytes = new Uint8Array(await shards[1].arrayBuffer())
+
+    // block 0 and 1 should be in shard 0
+    const slice0 = shards[0].slices.get(blocks[0].cid.multihash)
+    assert.ok(slice0)
+    assert(
+      equals(
+        blocks[0].bytes,
+        shard0Bytes.slice(slice0[0], slice0[0] + slice0[1])
+      )
+    )
+
+    const slice1 = shards[0].slices.get(blocks[1].cid.multihash)
+    assert.ok(slice1)
+    assert(
+      equals(
+        blocks[1].bytes,
+        shard0Bytes.slice(slice1[0], slice1[0] + slice1[1])
+      )
+    )
+
+    // block 2 should be in shard 1
+    const slice2 = shards[1].slices.get(blocks[2].cid.multihash)
+    assert.ok(slice2)
+    assert(
+      equals(
+        blocks[2].bytes,
+        shard1Bytes.slice(slice2[0], slice2[0] + slice2[1])
+      )
+    )
   })
 
   it('exceeds shard size when block is encoded with root CID', async () => {