尝试使用biopython创建基本的系统树，特别是遵循此文档页面https://biopython.org/DIST/docs/api/Bio.Phylo.TreeConstruction.DistanceTreeConstructor-class.html。每次我使用fastfa文件时，我似乎都会出错。

from Bio.Phylo.TreeConstruction import DistanceCalculator
aln = AlignIO.read("canine.fasta",'fasta')
constructor = DistanceTreeConstructor()
calculator = DistanceCalculator('identity')
dm = calculator.get_distance(aln)
upgmatree = constructor.upgma(dm)
print(upmgmatree) 

我得到的错误：

  File "/Users/theodorelatimer/Desktop/Interna Assment HL Biology/Bio IA.py",line 36,in <module>
    upgmatree = constructor.upgma(dm)
  File "/library/Frameworks/Python.framework/Versions/3.7/lib/python3.7/site-packages/Bio/Phylo/TreeConstruction.py",line 696,in upgma
    inner_clade.branch_length = 0
UnboundLocalError: local variable 'inner_clade' referenced before assignment

我知道在我链接的phylip的文档中使用过，但是我不确定如何进行序列比对。