Weird behaviour with parameters in texboxes...

I'm having a strange reaction from a simple expression in SSRS, or I might be the stupid part here :/

Here is the switch 

Switch(
Parameters!CustomerData.Count<=7,join(Parameters!CustomerData.Value,", "),

Parameters!CustomerData.Count>7 and Parameters!CustomerData.Count<Count(Fields!name_alt.Value, "Contacts"), Parameters!CustomerData.Value(0)& ", "&Parameters!CustomerData.Value(1)& ", "&Parameters!CustomerData.Value(2)& ", "&Parameters!CustomerData.Value(3)& ", "&Parameters!CustomerData.Value(4)& ", "&Parameters!CustomerData.Value(5)& ", "&Parameters!CustomerData.Value(6)& " (...)",

Parameters!CustomerData.Count=Count(Fields!name_alt.Value, "Contacts"),"All Customers Selected"
)

-- So when I have 7 or More selected on my parameter the label changes OK 

-- When I have ALL selected , also working tip top :)

-- When I have exactly 7 .... perfect !

-->> But if I have less than 7 selected in my Parameter ... I get #Error ... does anyone knows ???? I think I've written a good code :/

Thanks a lot in advance

Tiago