使用statistics.stdev()时出现'numpy.float64' object has no attribute 'numerator'错误的排查求助

我写了两个用来计算Cohen's D的函数，用于处理CSV文件的数据：

import statistics
import numpy as np

def cohen_d(a,b):
    float_a = []
    float_b = []
    for x in a:
        try:
            float_a.append(float(x))
        except ValueError:
            return "NAN"
    for y in b:
        try:
            float_b.append(float(y))
        except ValueError:
            return "NAN"
    try:
        res=(statistics.fmean(float_a) - statistics.fmean(float_b)) / statistics.stdev(float_a+float_b)
    except ZeroDivisionError:
        res=0
    return res

def cohen_d_np(a,b):
    float_a = []
    float_b = []
    for x in a:
        try:
            float_a.append(float(x))
        except ValueError:
            return "NAN"
    for y in b:
        try:
            float_b.append(float(y))
        except ValueError:
            return "NAN"
    float_a = np.array(float_a)
    float_b = np.array(float_b)
    try:
        res=(statistics.fmean(float_a) - statistics.fmean(float_b)) / statistics.stdev(np.concatenate([float_a,float_b]))
    except ZeroDivisionError:
        res=0
    return res

当我在自己的数据上运行这些函数时，基于NumPy的版本会触发如下错误，而基于列表的版本则会报错'float' object has no attribute 'numerator'：

RuleException:
AttributeError in file /work/user/cburnard/PIPELINES/ChIPseq/Snakefile, line 111:
'numpy.float64' object has no attribute 'numerator'
  File "/work/user/cburnard/PIPELINES/ChIPseq/Snakefile", line 903, in __rule_calculate_multimapbw_indiv_scores
  File "/work/user/cburnard/PIPELINES/ChIPseq/Snakefile", line 162, in effect_size_mmb
  File "/work/user/cburnard/PIPELINES/ChIPseq/Snakefile", line 111, in cohen_d_np
  File "/tools/devel/python/Python-3.11.1/lib/python3.11/statistics.py", line 922, in stdev
  File "/tools/devel/python/Python-3.11.1/lib/python3.11/concurrent/futures/thread.py", line 58, in run

这个错误是怎么产生的？我不确定具体是哪一行数据导致的，但理论上即使存在非数值数据的问题，函数也应该返回"NAN"，而不是抛出这个错误。

备注：内容来源于stack exchange，提问作者Whitehot