This happens if:
1. There is no Bin/ directory 
2. Assembly is in GAC
3. I add <%@ Assembly name="MySQL.Data" %> to the .asmx  file


System.NullReferenceException: Object reference not set to an instance of an object
  at System.Web.UI.SimpleWebHandlerParser.AddAssembly (System.Reflection.Assembly assembly, Boolean fullPath) [0x00000] 
  at System.Web.UI.SimpleWebHandlerParser.AddAssemblyByName (System.String name, ILocation location) [0x00000] 
  at System.Web.UI.SimpleWebHandlerParser.AddAssemblyDirective (ILocation location, System.Web.Compilation.TagAttributes attrs) [0x00000] 
  at System.Web.UI.SimpleWebHandlerParser.TagParsed (ILocation location, TagType tagtype, System.String tagid, System.Web.Compilation.TagAttributes attributes) [0x00000] 
  at System.Web.Compilation.AspParser.OnTagParsed (TagType tagtype, System.String id, System.Web.Compilation.TagAttributes attributes) [0x00000] 
  at System.Web.Compilation.AspParser.Parse () [0x00000] 
  at System.Web.UI.SimpleWebHandlerParser.ParseDirective (System.String line) [0x00000] 
  at System.Web.UI.SimpleWebHandlerParser.GetDirectivesAndContent () [0x00000] 
  at System.Web.UI.SimpleWebHandlerParser..ctor (System.Web.HttpContext context, System.String virtualPath, System.String physicalPath) [0x00000] 
  at System.Web.UI.WebServiceParser..ctor (System.Web.HttpContext context, System.String virtualPath, System.String physicalPath) [0x00000] 
  at System.Web.UI.WebServiceParser.GetCompiledType (System.String inputFile, System.Web.HttpContext context) [0x00000] 
  at System.Web.Services.Protocols.WebServiceHandlerFactory.GetHandler (System.Web.HttpContext context, System.String verb, System.String url, System.String filePath) [0x00000] 
  at System.Web.HttpApplication.GetHandler (System.Web.HttpContext context) [0x00000] 
  at System.Web.HttpApplication+<>c__CompilerGenerated1.MoveNext () [0x00000]