It's because you're using the "+" operator, which is addition. Addition means concatenation when both operands are strings but if one is a number then it means numerical addition and the non-numeric operand will be implicitly converted to a number. That conversion is failing in your case, as you'd expect. The "proper" string concatenation operator is "&". That will implicitly convert numbers to strings.

Also, you're using parameters for three values and then you go and use a literal value for the fourth. You should be using a parameter for Mem_Id too.