Assume that f : D 1 → R and g : D 2 → R are uniformly continuous functions, where D 1 , D 2 ⊂ X are nonempty open and arc-connected subsets of a real normed space X. We prove that then either f and g are affine functions, that is f (x) = x * (x) + a and g(x) = x * (x) + b with some x * ∈ X * and a, b ∈ R or the algebraic sum of graphs of functions f and g has a nonempty interior in a product space X × R treated as a normed space with a norm (x, α) =x 2 + |α| 2 .