The error occurs when evalf or convert are passed expressions that possibly contain multiple roots.

Example 1
 

The actual roots of $x^2-x-1$are $\left\{\frac{1}{2}\+\frac{\sqrt{5}}{2}\, \frac{1}{2}-\frac{\sqrt{5}}{2}\right\}\.$

Example 2

The actual roots of $x^2-x-2$are $\left\{2 \, -1\right\}\.$

examples,RootOf