python ValueError: could not convert string to float: ? 该怎么解决

#coding:utf-8
import numpy as np
import pandas as pd
from sklearn.ensemble import IsolationForest
ilf = IsolationForest(n_estimators=100,
                      n_jobs=-1,          # 使用全部cpu
                      verbose=2,
    )
data = pd.read_excel('data.xlsx',index_col='AA')
data = data.fillna(0)
# 选取特征，不使用标签(类型)
X_cols = ["BB", "CC"]
print data.shape

# 训练
ilf.fit(data[X_cols])
shape = data.shape[0]
batch = 10**6

all_pred = []
for i in range(shape/batch+1):
    start = i * batch
    end = (i+1) * batch
    test = data[start:end]
    # 预测
    pred = ilf.predict(test)
    all_pred.extend(pred)

data['pred'] = all_pred
data.to_excel('outliers.xlsx', columns=["pred",], header=False)

得到的结果

"D:\ProgramData\New Folder\python.exe" C:/Users/ZS/PycharmProjects/untitled/AS
(23648, 6)
Traceback (most recent call last):
  File "C:/Users/ZS/PycharmProjects/untitled/AS", line 16, in 
    ilf.fit(data[X_cols])
  File "C:\Users\ZS\AppData\Roaming\Python\Python27\site-packages\sklearn\ensemble\iforest.py", line 162, in fit
    X = check_array(X, accept_sparse=['csc'])
  File "C:\Users\ZS\AppData\Roaming\Python\Python27\site-packages\sklearn\utils\validation.py", line 448, in check_array
    array = array.astype(np.float64)
ValueError: could not convert string to float: ?


Process finished with exit code 1