Fail with Shrine::Error when storage is not S3

lib/shrine/plugins/uppy_s3_multipart.rb

@@ -11,6 +11,10 @@ class Shrine
         def uppy_s3_multipart(storage_key, **options)
           s3 = find_storage(storage_key)
 
+          unless defined?(Shrine::Storage::S3) && s3.is_a?(Shrine::Storage::S3)
+            fail Error, "expected storage to be a Shrine::Storage::S3, but was #{s3.inspect}"
+          end
+
           ::Uppy::S3Multipart::App.new(
             bucket:  s3.bucket,
             prefix:  s3.prefix,

test/shrine_test.rb

 require "test_helper"
+
 require "shrine"
 require "shrine/plugins/uppy_s3_multipart"
 require "shrine/storage/s3"
+require "shrine/storage/memory"
+
 require "rack/test_app"
 
 describe Shrine::Plugins::UppyS3Multipart do
@@ -69,4 +72,19 @@ describe Shrine::Plugins::UppyS3Multipart do
     assert_equal :create_multipart_upload, client.api_requests[0][:operation_name]
     assert_equal "public-read",            client.api_requests[0][:params][:acl]
   end
+
+  it "works for subclass of S3 storage" do
+    @shrine.storages[:s3] = Class.new(Shrine::Storage::S3).new(
+      bucket:         "my-bucket",
+      stub_responses: true,
+    )
+
+    @shrine.uppy_s3_multipart(:s3)
+  end
+
+  it "fails for non-S3 storage" do
+    @shrine.storages[:memory] = Shrine::Storage::Memory.new
+
+    assert_raises(Shrine::Error) { @shrine.uppy_s3_multipart(:memory) }
+  end
 end

uppy-s3_multipart.gemspec

@@ -20,5 +20,6 @@ Gem::Specification.new do |gem|
   gem.add_development_dependency "minitest"
   gem.add_development_dependency "rack-test_app"
   gem.add_development_dependency "shrine", "~> 2.0"
+  gem.add_development_dependency "shrine-memory"
   gem.add_development_dependency "aws-sdk-core", "~> 3.23"
 end