WCF Error : Index was out of range. Must be non-negative and less than the size of the collection. Parameter name: index

"Server Error in '/app' Application.
Index was out of range. Must be non-negative and less than the size of the collection.
Parameter name: index
Description: An unhandled exception occurred during the execution of the current web request. Please review the stack trace for more information about the error and where it originated in the code.

Exception Details: System.ArgumentOutOfRangeException: Index was out of range. Must be non-negative and less than the size of the collection.
Parameter name: index

Source Error:

An unhandled exception was generated during the execution of the current web request. Information regarding the origin and location of the exception can be identified using the exception stack trace below.

Stack Trace:


[ArgumentOutOfRangeException: Index was out of range. Must be non-negative and less than the size of the collection.
Parameter name: index]
   System.ThrowHelper.ThrowArgumentOutOfRangeException(ExceptionArgument argument, ExceptionResource resource) +64
   System.ThrowHelper.ThrowArgumentOutOfRangeException() +15
   System.Collections.Generic.List`1.get_Item(Int32 index) +7504692
   System.ServiceModel.Web.WebServiceHost.AddAutomaticWebHttpBindingEndpoints(ServiceHost host, IDictionary`2 implementedContracts, String multipleContractsErrorMessage) +82
   System.ServiceModel.Web.WebServiceHost.OnOpening() +203
   System.ServiceModel.Channels.CommunicationObject.Open(TimeSpan timeout) +229
   System.ServiceModel.HostingManager.ActivateService(String normalizedVirtualPath) +121
   System.ServiceModel.HostingManager.EnsureServiceAvailable(String normalizedVirtualPath) +479

[ServiceActivationException: The service '/app/JSON/UserService.svc' cannot be activated due to an exception during compilation.  The exception message is: Index was out of range. Must be non-negative and less than the size of the collection.
Parameter name: index.]
   System.ServiceModel.AsyncResult.End(IAsyncResult result) +11654478
   System.ServiceModel.Activation.HostedHttpRequestAsyncResult.End(IAsyncResult result) +194
   System.ServiceModel.Activation.HostedHttpRequestAsyncResult.ExecuteSynchronous(HttpApplication context, Boolean flowContext) +176
   System.ServiceModel.Activation.HttpModule.ProcessRequest(Object sender, EventArgs e) +275
   System.Web.SyncEventExecutionStep.System.Web.HttpApplication.IExecutionStep.Execute() +68
   System.Web.HttpApplication.ExecuteStep(IExecutionStep step, Boolean& completedSynchronously) +75"