Problem with multline - missing \right)

Question

I have put the following formula in a latex document but I am getting an error missing \right.

\begin{multline}
    \int\;\dd^3\vec{x}\int\frac{\dd^3\vec{k^\prime}}{2E(\vec{k}^\prime)(2\pi)^3}
    \left(
        (2\pi)^3\dirac{3}{\vec{k}^\prime-\vec{k}}
        \e{i\dots}a^\dagger(\vec{k}^\prime)\\
        +(2\pi)^3\dirac{3}{\vec{k}^\prime-\vec{k}}
        \e{i\dots}a(\vec{k}^\prime)
    \right)
\end{multline}

Note that if I remove the \\ then it works fine, ie;

\begin{multline}
    \int\;\dd^3\vec{x}\int\frac{\dd^3\vec{k^\prime}}{2E(\vec{k}^\prime)(2\pi)^3}
    \left(
        (2\pi)^3\dirac{3}{\vec{k}^\prime-\vec{k}}
        \e{i\dots}a^\dagger(\vec{k}^\prime)
        +(2\pi)^3\dirac{3}{\vec{k}^\prime-\vec{k}}
        \e{i\dots}a(\vec{k}^\prime)
    \right)
\end{multline}

That ^^ works.