RecursionError when calling fastcore.basics.concat([["abc", "xyz"], ["foo", "bar"]])

[Ark-kun]

fastcore.basics.concat([["abc", "xyz"], ["foo", "bar"]])

----> 2 fastcore.basics.concat([["abc", "xyz"], ["foo", "bar"]])

/opt/conda/lib/python3.7/site-packages/fastcore/basics.py in concat(colls)
    431 def concat(colls)->list:
    432     "Concatenate all collections and items as a list"
--> 433     return list(flatten(colls))
    434 
    435 # Cell

/opt/conda/lib/python3.7/site-packages/fastcore/basics.py in flatten(o)
    425     "Concatenate all collections and items as a generator"
    426     for item in o:
--> 427         try: yield from flatten(item)
    428         except TypeError: yield item
    429 

... last 1 frames repeated, from the frame below ...

/opt/conda/lib/python3.7/site-packages/fastcore/basics.py in flatten(o)
    425     "Concatenate all collections and items as a generator"
    426     for item in o:
--> 427         try: yield from flatten(item)
    428         except TypeError: yield item
    429 

RecursionError: maximum recursion depth exceeded