Sum Sql column with join

I'm trying this code:

Select  C.CustomerNum
      , C.Coupon
      , C.name
      , C.Surname
      , Sum(P.Points)
From    customers C
Join    Points P
        On P.CustomerNum = C.CustomerNum
Where   C.Coupon = 'xxx-xxx-xxx-x'; 

I'm getting error:

Msg 8118, Level 16, State 1, Line 1 Column 'C.CustomerNum' is invalid in the select list because it is not contained in an aggregate function and there is no GROUP BY clause.