Unable to port custom ContentTypeWriter to MonoGame 3.6

I’ve created a custom model processor library/pipeline, that needs a custom ContentTypeWriter for EffectMaterialContent:

using System.Collections.Generic;
using Microsoft.Xna.Framework.Content.Pipeline.Serialization.Compiler;
using Microsoft.Xna.Framework.Content;
using Microsoft.Xna.Framework.Content.Pipeline;
using Microsoft.Xna.Framework.Content.Pipeline.Graphics;

namespace JuliPipeline
{
    [ContentTypeWriter]
    public class EffectMaterialWriter1 : ContentTypeWriter<EffectMaterialContent>
    {
        protected override void Write(ContentWriter output, EffectMaterialContent value)
        {
            output.WriteExternalReference(value.CompiledEffect);
            Dictionary<string, object> dict = new Dictionary<string, object>();
            foreach (KeyValuePair<string, ExternalReference<TextureContent>> item in value.Textures)
            {
                dict.Add(item.Key, item.Value);
            }

            foreach (var item in value.OpaqueData)
            {
                if (item.Key != EffectMaterialContent.EffectKey && item.Key != EffectMaterialContent.CompiledEffectKey)
                    dict.Add(item.Key, item.Value);
            }

            output.WriteObject<Dictionary<string, object>>(dict);
        }

        public override string GetRuntimeReader(TargetPlatform targetPlatform)
        {
            var type = typeof(ContentReader);
            var readerType = type.Namespace + ".EffectMaterialReader, " + type.Assembly.FullName;
            return readerType;
        }
    }
}

When I’m referencing this dll in the pipeline editor, I’m unable to build any(!) content. It sais translated
“An element with the same key has already been added”.
By excluding this class from the dll I can build content again, but I’m unable to load my models with a custom effect, because the wrong runtimeReader is loaded(?). This worked perfectly in MG3.5, but now something changed and I can’t find the source of the problem. Does anybody have the same problem or know how to fix it?