example:

/fill -8 202 6 -5 202 6 minecraft:unpowered_comparator 0 replace stone

->

Data tag parsing failed: Invalid tag encountered, expected '{' as first char.

But:

/fill -8 202 6 -5 202 6 minecraft:unpowered_repeater 0 replace stone

and

/fill -8 202 6 -5 202 6 minecraft:unpowered_comparator

works.