Why am I unable to do:

dict = { **sub_dict for sub_dict in super_dict.values() }

which throws: "dict unpacking cannot be used in dict comprehension"

Whereas the equivalent code below doesn't throw any errors?:

dict = {}
for sub_dict in super_dict.values():
    dict = { **dict, **sub_dict }

Am I wrong in thinking the first and second block of code should do the same thing behind the scenes?