diff --git a/CHANGELOG.md b/CHANGELOG.md
index 57829da..10311ee 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -1,5 +1,6 @@
 ### 3.2.3 (Next)
 
+* [#57](https://github.com/mongoid/mongo_session_store/pull/57): Cover legacy session decoding and version loading, and fix private decoder delegation in the plain-driver adapter - [@dblock](https://github.com/dblock).
 * [#55](https://github.com/mongoid/mongo_session_store/pull/55): Add explicit bulk stale-session cleanup with an application-selected cutoff for both adapters - [@dblock](https://github.com/dblock).
 * [#54](https://github.com/mongoid/mongo_session_store/pull/54): Document and test storing permitted parameter hashes rather than request-bearing Rails parameter objects in sessions - [@dblock](https://github.com/dblock).
 * [#53](https://github.com/mongoid/mongo_session_store/pull/53): Cover Rails 6.1 through 8.0 in CI, update RuboCop, isolate Danger permissions, and report both adapters' coverage to Coveralls - [@dblock](https://github.com/dblock).
diff --git a/gemfiles/rails-current-mongo.gemfile b/gemfiles/rails-current-mongo.gemfile
index 4290326..9d39e9e 100644
--- a/gemfiles/rails-current-mongo.gemfile
+++ b/gemfiles/rails-current-mongo.gemfile
@@ -11,5 +11,6 @@ gem "drb"
 gem "base64"
 gem "benchmark"
 gem "ostruct"
+gem "simplecov", "~> 0.22", :require => false if ENV["COVERAGE"] == "true"
 
 gemspec :path => "../"
diff --git a/lib/mongo_session_store/mongo_store.rb b/lib/mongo_session_store/mongo_store.rb
index 77e43f3..f2b0f76 100644
--- a/lib/mongo_session_store/mongo_store.rb
+++ b/lib/mongo_session_store/mongo_store.rb
@@ -92,7 +92,7 @@ def pack(data)
         end
 
         def unpack(packed)
-          self.class.unpack(packed)
+          self.class.send(:unpack, packed)
         end
       end
     end
diff --git a/spec/lib/mongo_session_store/legacy_serialization_spec.rb b/spec/lib/mongo_session_store/legacy_serialization_spec.rb
new file mode 100644
index 0000000..b8daebd
--- /dev/null
+++ b/spec/lib/mongo_session_store/legacy_serialization_spec.rb
@@ -0,0 +1,17 @@
+require "spec_helper"
+
+describe "Legacy session serialization" do
+  let(:session_class) { mongo_orm == "mongoid" ? MongoidStore::Session : MongoStore::Session }
+
+  it "reads raw marshalled strings as well as BSON binary data" do
+    payload = { "nested" => { "count" => 2 }, "tags" => %w[old session] }
+    session = session_class.new
+
+    expect(session.send(:unpack, Marshal.dump(payload))).to eq(payload)
+    expect(session.send(:unpack, BSON::Binary.new(Marshal.dump(payload), :generic))).to eq(payload)
+  end
+
+  it "returns nil when there is no serialized payload" do
+    expect(session_class.new.send(:unpack, nil)).to be_nil
+  end
+end
diff --git a/spec/lib/mongo_session_store/version_spec.rb b/spec/lib/mongo_session_store/version_spec.rb
new file mode 100644
index 0000000..581d0d9
--- /dev/null
+++ b/spec/lib/mongo_session_store/version_spec.rb
@@ -0,0 +1,13 @@
+require "spec_helper"
+
+describe "Version entry point" do
+  it "defines a frozen semantic version when loaded on its own" do
+    hide_const("MongoSessionStore::VERSION")
+    load File.expand_path("../../../lib/mongo_session_store/version.rb", __dir__)
+
+    expect(MongoSessionStore::VERSION).to match(/\A\d+\.\d+\.\d+\z/)
+    expect(MongoSessionStore::VERSION).to be_frozen
+  ensure
+    MongoSessionStore.send(:remove_const, :VERSION) if MongoSessionStore.const_defined?(:VERSION)
+  end
+end