I want to replace an empty dispenser in a specific area with a dispenser with Items in it. But the position of the dispenser varies each time. So i thought i could solve this via /fill and replace all found dispensers with a new one.
To short it down i tested if it works when i only want to exchange dispenser by dispensers (without stuff in it)

As an example this works (replace stained clay with a redstone block):

/fill ~8 81 ~8 ~-8 94 ~-8 stained_hardened_clay 6 replace redstone_block

And this fails

/fill ~8 81 ~8 ~-8 94 ~-8 dispenser 6 replace dispenser

with the error:

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

EDIT: After a few more testings: i found out that the first "dispenser" is causing the problem. When replacing it with another block e.g wool it works. So here seems to be the Issue

EDIT 2: Tested it with other "container blocks" like dropper, chests, Enderchest. Always the same Issue.

What do i do wrong or is there another way?