System.ServiceModel.FaultException'1' Where might the error be?

Question

I am calling a function in a wcf service which takes a string (of in-determinant length) as a parameter. This is a function that I have had working before (but admittedly in a much earlier version of visual studio).

When I now call this in a new version of the program built in vs 2013 using .net4.5.1 I get this ServiceModel faultexception telling me that 'String must be exactly one character long.

I know that the service definition has not changed and is as it was. However it is no longer possible for me to liase with the people who created it so I cannot quiz them. I am assuming that there is an error an my end that must be causing this but really don't know enough about this field to make a reasoned educated guess.

So two questions.

Am I right in my assumption that this is probably something wrong at my end? If not then I Using this service is going to be a waste of time.

If my assumption is correct and somewhere I have made an error where might that be. Do I for example need to add some extra configuration settings?

IF there is additional info I could provide but haven't please don't hesitate to ask. As I say this is an area of programing where in truth I'm still learning my way around so instinct suggests I'm at fault I just don't know why. I'm using similar code to that in a previous version which did work.

I know that I can actively interact with the service because I can pull info from it. The problem seems to be when sending info up to it.

Thanks

EDIT

this is the function I'm calling that produces the serviceModel exception;

Public Function salesXMLCheck(ByVal xmlString As String) As String Implements ErsTestSite.IERSAPIService.salesXMLCheck
            Return MyBase.Channel.salesXMLCheck(xmlString)
        End Function

and it's called from here;

 Using ters As New ErsTestSite.ERSAPIServiceClient
            ters.Open()
            returnsEditor.Text = ters.salesXMLAdd(submissionsEditor.Text)
            ters.Close()
        End Using

this is the config section for the service;

<bindings>
            <basicHttpBinding>
              <binding name="BasicHttpBinding_IERSAPIService"
                         maxReceivedMessageSize="20000000"
               maxBufferSize="20000000"
               maxBufferPoolSize="20000000">
                <security mode="None" />
              </binding>
            </basicHttpBinding>
        </bindings>