Actually, I've had a good mess about with 'coerce' and here is how it is behaving in 5.5:
indexing a value of "1" will succeed. Indexing a value of 1.1 will succeed. However, "1.0" (or "1.any digit") will fail.
You can see from the relevant code block that this is how it is coded to work (if the value is a string, coerce is apparently ignored altogether, it just tries to parse it)....
I'm not certain that is right, so I have opened this issue to suss it out 