  For a number field $K$ defined by a trinomail $F(x) = x^7+ax+b \in
\mathbb{Z}[x]$, Jakhar and Kumar gave some necessary conditions on $a$ and $b$,
which guarantee the non-monogenity of $K$ \cite{A6}. In this paper, for every
prime integer $p$, we characterize when $p$ is a common index divisor of $K$.
In particular, if any one of these conditions holds, then $K$ is not monogenic.
In such a way our proposed results extend those of Jakhar and Kumar.
