[Python] OSError: [Errno 8] Exec format error: './FastTree' [duplicate]

I keep getting this error which by my google search means that there is a shabang missing but I'm not sure where I"m supposed to put it. Any help is appreciated!

if tree_method == 'FastTree': # Build the tree using FastTree
command = ["./FastTree", path_msa]
os.makedirs(dir_tree + 'FastTree/', exist_ok=True)
path_tree = dir_tree + 'FastTree/' + filename_without_extension + '.newick'
with open(path_tree, "w") as outfile:
subprocess.call(command, stdout=outfile)


The error gets triggered on the last line of this code.

PS.When I run the same code from shell on linux as > FastTree <directory> it works just fine.

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