diff --git a/ext/java/org/msgpack/jruby/Factory.java b/ext/java/org/msgpack/jruby/Factory.java
index 34a6c22a..bd31e685 100644
--- a/ext/java/org/msgpack/jruby/Factory.java
+++ b/ext/java/org/msgpack/jruby/Factory.java
@@ -112,7 +112,7 @@ public IRubyObject registerTypeInternal(ThreadContext ctx, IRubyObject type, IRu
     extensionRegistry.put(extModule, (int) typeId, recursive, packerProc, unpackerProc);
 
     if (extModule == runtime.getSymbol()) {
-      hasSymbolExtType = !packerProc.isNil();
+      hasSymbolExtType = packerProc != null && !packerProc.isNil();
     }
 
     if (options != null) {
diff --git a/spec/factory_spec.rb b/spec/factory_spec.rb
index 15b5e5d3..2abedab3 100644
--- a/spec/factory_spec.rb
+++ b/spec/factory_spec.rb
@@ -273,6 +273,12 @@ class MyType2 < MyType
       expect(factory.load(factory.dump(:foo))).to be == "foo"
     end
 
+    it 'handles Symbol type registered with only an unpacker' do
+      factory = MessagePack::Factory.new
+      factory.register_type(0x00, Symbol, unpacker: :to_sym.to_proc)
+      expect(factory.load(factory.dump(:foo))).to be == "foo"
+    end
+
     describe "registering multiple ext type for the same class" do
       let(:payload) do
         factory = MessagePack::Factory.new