Which of the following statements is not true?

Choose the incorrect statement below.

A$.$ Because the imaginary unit $i$ is not a real number, it is impossible to evaluate an expression such as $i^n$ if $n$ is a negative integer.

B$.$ The imaginary unit, $i,$ is defined as $i=\sqrt[\phantom{2}]{-1}.$

C$.$ If $i$ is the imaginary unit, then $i^2=-1.$

D$.$ The imaginary unit, $i,$ is a solution to the equation $x^2+1=0.$