Procedure:
fill ~ ~ ~ ~ ~ ~ command_block 0 replace air

Expected:
The placement of a command block at player's position.

Results:
No blocks are placed and an error message : Data tag parsing failed: Invalid tag encountered, expected '{' as first char.

Additional information

When executed as: /fill ~ ~ ~ ~ ~ ~ minecraft:command_block 0 0 {} replace air it even spells the error message wrongly (misses the last character) : _Data tag parsing failed: Unable to locate name/value separator for string: } replace ai.

[x] It works only if not replacing : fill ~ ~ ~ ~ ~ ~ command_block 0

Applying fill on large areas to replicate data blocks is an important feature.