Why does MAXUPVALS apply to both Lua and C closures?

[Hugo Musso Gualandi <hgualandi@...>]

PUC Lua 5.4 has a hard limit of 255 upvalues, both for Lua closures and 
for C closures. The comment next to MAXUPVALS in lfunc.c says that this 
is because the index of an upvalue must fit in an 8-bit virtual-machine 
register.

I understand why this would be the case for Lua closures. Many VM 
instructions such as GETUPVAL refer to the upvalue using an 8 bit 
address.

However, I don't understand why this limit also applies to C closures, 
the ones created via the C API with lua_pushcclosure. Is there some 
place where the number of upvalues of a C closure must fit in 8 bits? 
Or is this limit only to ensure that the limit is similar to the limit 
for Lua closures?

-- Hugo