We show that a weaker version of the well-known theorem of Morlaye and Joly on diagonal equations is a simple consequence of a restricted variable version of the Chevalley-Warning theorem. Moreover, we extend the result of Morlaye and Joly to the case of an equation of the form b 1 D m1 (X 1 , a 1 ) + · · · + b n D mn (X n , a n ) = c, where D m1 (X 1 , a 1 ), . . . , D mn (X n , a n ) are Dickson polynomials.