如何取消对替换的spacy.tokens.token.Token的标记？

import spacy

nlp = spacy.load("en_core_web_sm")
doc = nlp("This is a sentence.")

# 替换第一个token的标记
doc[0].dep_ = ""

# 输出替换后的结果
for token in doc:
    print(token.text, token.dep_)

This 
is ROOT
a det
sentence nsubj
. punct