The error occurs when evalf or convert are passed expressions that have ambiguity in regards to specification of roots.

Example 1

The numerical approximation of $\frac{1}{2}$used as a root selector is not helpful in distinguishing between the two roots, $\frac{1}{2}-\frac{\sqrt{5}}{2}\, \frac{1}{2}\+\frac{\sqrt{5}}{2}\,$since both roots are a distance of exactly $\frac{\sqrt{5}}{2}$from $\frac{1}{2}\.$

Solution

Use a different value as the root selector:

Example 2

The numeric selector, $0$, is not sufficient to distinguish whether $r_1$ is the positive or negative root. Therefore, the following gcd computation is ambiguous: if $r_1$ represents the positive root, then the GCD is $x-\sqrt{2}$, but if $r_1$ represents the negative root, then the GCD is 1. Hence an error is raised:

Solution

If you use a different selector that is not ambiguous, you can get an answer. Here, we use index=1 and index=2.

We've specified two distinct roots, and their gcd is  1.

Example 3

Solution

This is the same root selector problem as in example 1.  As was done for that example, by choosing a different numeric selector for the RootOf, the specification is no longer ambiguous: