We discusses the charge asymmetry in B →DD¯ K decays with an invariant mass of the DD¯ pair near the Ψ(3770) resonance. Unlike Ψ(3770) decays in e+e− annihilation, in B+ decays the probability of DD¯ production is almost three times higher than D+D−. In B0 decays, the ratio of these probabilities will be opposite. The effect is explained by the fact that, in B-meson decays, the DD¯ pair is produced in a superposition of isoscalar and isovector states, and only in combination with K-mesons the total state has 1/2 isospin. We present a simple model in which the interference of the nonresonant isovector amplitude with the resonant isoscalar amplitude explains the experimental data.