TypeError expected in an augmented assignment

[candide]

An hybrid list-tuple concatenation is not allowed

>>> []+(1, 2)
Traceback (most recent call last):                                                                        
  File "<stdin>", line 1, in <module>                                                                     
TypeError: can only concatenate list (not "tuple") to list                                                
>>>  



hence I was expecting (*) that the following code raises a TypeError :

>>> x = []                                                                                                
>>> x += (1, 2)                                                                                           
>>> x                                                                                                     
[1, 2]                                                                                                    
>>>


Any explanation ?


(*) as the docs states, the augmented assignment is supposed to perform the concatenation : 
An augmented assignment (...) performs the binary operation specific to the type of assignment on the two operands (...)