### negative number cannot be raised to a fractional power in RDF

Hi, can anyone tell me why this evaluation is correct:

```
var('y')
h(x,y) = sqrt(1-(cos(2*y)*sin(2*x)))
G(x,y) = sum([comp^2 for comp in diff(h)])
```~~diff(G)(*vector((0.1,0.75)))
~~diff(G)(*vector(RR,(0.1,0.75)))

(0.788319732748083,
-0.281637345440034)

But this evaluation, using RDF, is not correct:

```
diff(G)(*vector(RDF,(0.1,0.75)))
```

ValueError: negative number cannot be
raised to a fractional power

I am aware that the error is related with the question https://ask.sagemath.org/question/7820/fractional-power-to-negative-number/, but I would expect the same behavior in both cases, with and without RDF.