If I use the commands:
– say @e[type=!Player,c=1]
– say @e[type=!Player,c=2]
while I have a few other entities in my world I get their names as output into the chat as expected.

While the dummy objective "o" exists, if I do:
– scoreboard players set @e[type=!Player,c=1] o 1
I get: "That player cannot be found"
Whereas:
– scoreboard players set @e[type=!Player,c=2] o 1
works like intended and successfully sets the appropriate two scores.

According to my testing so far the same error applies for:
– scoreboard players add @e[type=!Player,c=1] o 1
– scoreboard players remove @e[type=!Player,c=1] o 1
– scoreboard players operation @e[type=!Player,c=1] o += a o
as for *-=, =, */= and %= of course.
Also with anything else for type:
– say @e[type=MinecartRideable,c=1]