For a two-particle system, prove that $\langle m_1, m_2 | j, m \rangle \neq 0$ where $m = m_1 + m_2$. Here $m_1$ and $m_2$ are magnitudes of $Z$-components of total angular moments of individual particle systems, $j$ and $m$ are total angular momentum and $Z$-component of the total angular momenta of the composite system of two particles.