diff --git a/src/streaming/s3.sink.cpp b/src/streaming/s3.sink.cpp
index 04ea3c7..81150ab 100644
--- a/src/streaming/s3.sink.cpp
+++ b/src/streaming/s3.sink.cpp
@@ -128,17 +128,13 @@ zarr::S3Sink::is_multipart_upload_() const
 void
 zarr::S3Sink::create_multipart_upload_()
 {
-    if (!is_multipart_upload_()) {
-        multipart_upload_ = {};
-    }
-
-    if (!multipart_upload_->upload_id.empty()) {
-        return;
-    }
+    multipart_upload_ = MultiPartUpload{};
 
+    auto connection = connection_pool_->get_connection();
     multipart_upload_->upload_id =
-      connection_pool_->get_connection()->create_multipart_object(bucket_name_,
-                                                                  object_key_);
+      connection->create_multipart_object(bucket_name_, object_key_);
+
+    connection_pool_->return_connection(std::move(connection));
 }
 
 bool
@@ -148,9 +144,11 @@ zarr::S3Sink::flush_part_()
         return false;
     }
 
-    auto connection = connection_pool_->get_connection();
+    if (!is_multipart_upload_()) {
+        create_multipart_upload_();
+    }
 
-    create_multipart_upload_();
+    auto connection = connection_pool_->get_connection();
 
     bool retval = false;
     try {
diff --git a/tests/unit-tests/CMakeLists.txt b/tests/unit-tests/CMakeLists.txt
index aefbc34..6069b73 100644
--- a/tests/unit-tests/CMakeLists.txt
+++ b/tests/unit-tests/CMakeLists.txt
@@ -14,6 +14,7 @@ set(tests
         s3-connection-upload-multipart-object
         file-sink-write
         s3-sink-write
+        s3-sink-write-multipart
         sink-creator-make-metadata-sinks
         sink-creator-make-data-sinks
         array-writer-downsample-writer-config
diff --git a/tests/unit-tests/s3-sink-write-multipart.cpp b/tests/unit-tests/s3-sink-write-multipart.cpp
new file mode 100644
index 0000000..a335281
--- /dev/null
+++ b/tests/unit-tests/s3-sink-write-multipart.cpp
@@ -0,0 +1,112 @@
+#include "s3.sink.hh"
+#include "unit.test.macros.hh"
+
+#include <cstdlib>
+#include <memory>
+
+namespace {
+bool
+get_credentials(std::string& endpoint,
+                std::string& bucket_name,
+                std::string& access_key_id,
+                std::string& secret_access_key)
+{
+    char* env = nullptr;
+    if (!(env = std::getenv("ZARR_S3_ENDPOINT"))) {
+        LOG_ERROR("ZARR_S3_ENDPOINT not set.");
+        return false;
+    }
+    endpoint = env;
+
+    if (!(env = std::getenv("ZARR_S3_BUCKET_NAME"))) {
+        LOG_ERROR("ZARR_S3_BUCKET_NAME not set.");
+        return false;
+    }
+    bucket_name = env;
+
+    if (!(env = std::getenv("ZARR_S3_ACCESS_KEY_ID"))) {
+        LOG_ERROR("ZARR_S3_ACCESS_KEY_ID not set.");
+        return false;
+    }
+    access_key_id = env;
+
+    if (!(env = std::getenv("ZARR_S3_SECRET_ACCESS_KEY"))) {
+        LOG_ERROR("ZARR_S3_SECRET_ACCESS_KEY not set.");
+        return false;
+    }
+    secret_access_key = env;
+
+    return true;
+}
+} // namespace
+
+int
+main()
+{
+    std::string s3_endpoint, bucket_name, s3_access_key_id,
+      s3_secret_access_key;
+
+    if (!get_credentials(
+          s3_endpoint, bucket_name, s3_access_key_id, s3_secret_access_key)) {
+        LOG_WARNING("Failed to get credentials. Skipping test.");
+        return 0;
+    }
+
+    int retval = 1;
+    const std::string object_name = "test-object";
+
+    try {
+        auto pool = std::make_shared<zarr::S3ConnectionPool>(
+          1, s3_endpoint, s3_access_key_id, s3_secret_access_key);
+
+        auto conn = pool->get_connection();
+        if (!conn->is_connection_valid()) {
+            LOG_ERROR("Failed to connect to S3.");
+            return 1;
+        }
+        CHECK(conn->bucket_exists(bucket_name));
+        CHECK(conn->delete_object(bucket_name, object_name));
+        CHECK(!conn->object_exists(bucket_name, object_name));
+
+        pool->return_connection(std::move(conn));
+
+        std::vector<std::byte> data((5 << 20) + 1, std::byte{ 0 });
+        {
+            auto sink =
+              std::make_unique<zarr::S3Sink>(bucket_name, object_name, pool);
+            CHECK(sink->write(0, data));
+            CHECK(zarr::finalize_sink(std::move(sink)));
+        }
+
+        conn = pool->get_connection();
+        CHECK(conn->object_exists(bucket_name, object_name));
+        pool->return_connection(std::move(conn));
+
+        // Verify the object size.
+        {
+            minio::s3::BaseUrl url(s3_endpoint);
+            url.https = s3_endpoint.starts_with("https://");
+
+            minio::creds::StaticProvider provider(s3_access_key_id,
+                                                  s3_secret_access_key);
+
+            minio::s3::Client client(url, &provider);
+            minio::s3::StatObjectArgs args;
+            args.bucket = bucket_name;
+            args.object = object_name;
+
+            minio::s3::StatObjectResponse resp = client.StatObject(args);
+            EXPECT_EQ(int, data.size(), resp.size);
+        }
+
+        // cleanup
+        conn = pool->get_connection();
+        CHECK(conn->delete_object(bucket_name, object_name));
+
+        retval = 0;
+    } catch (const std::exception& e) {
+        LOG_ERROR("Exception: ", e.what());
+    }
+
+    return retval;
+}
\ No newline at end of file