Crm 2011 C# custom workflow

I’ve created a blank workflow in C# just for testing but I’m getting an error that I don’t understand and can’t find anything wrong with it. Any ideas?

Error

Unhandled Exception: System.ServiceModel.FaultException`1[[Microsoft.Xrm.Sdk.OrganizationServiceFault, Microsoft.Xrm.Sdk, Version=5.0.0.0, Culture=neutral, PublicKeyToken=31bf3856ad364e35]]: Workflow contains an invalid custom activity.Detail: 
<OrganizationServiceFault xmlns:i="http://www.w3.org/2001/XMLSchema-instance" xmlns="http://schemas.microsoft.com/xrm/2011/Contracts">
  <ErrorCode>-2147200968</ErrorCode>
  <ErrorDetails xmlns:d2p1="http://schemas.datacontract.org/2004/07/System.Collections.Generic" />
  <Message>Workflow contains an invalid custom activity.</Message>
  <Timestamp>2013-05-21T12:48:15.9172776Z</Timestamp>
  <InnerFault i:nil="true" />
  <TraceText i:nil="true" />
</OrganizationServiceFault>