Let K be a finite extension of the p-adic field Q_p and let F(X,Y) and G(X,Y) be one-dimensional formal group laws over the ring of integers O_K of K. Let φ(X) be a homomorphism from F to G which is defined over the ring of integers O_Cp of the completion C_p of Q_p ^alg. In this paper we prove that if ker(φ) is finite then there is a discretely valued subfield L⊂C_p such that φ(X) is defined over O_L.