TypeError：数组dtype（'<U32'）与格式说明符（'％.18e'）之间不匹配

I am using np.savetxt for the first time, and I am trying to save two variables (a string and a float) in a file named "trial.csv" as follows:

import numpy as np

RT = 2.76197329736740
key_name = 'space'

print(RT,key_name)
# Save data in a CSV file named subj_data_file
np.savetxt("trial.csv", (RT,key_name), delimiter=',', header="RTs,Key_Name")

我收到以下错误：

TypeError: Mismatch between array dtype ('<U32') and format specifier ('%.18e')

I do not understand the meaning of both ('<U32') and ('%.18e'). As a matter of fact, I do not understand how to use fmt when I have floats, integers and strings ...

这是一个简化的示例，但具体来说，我将在“ RTs”列中包含RT值（浮点数），而在另一列“ Key_Name”中具有key_name（浮点数）值。稍后，我将创建更多列，尽管在此示例中我为RT提供了一个值，为key_name提供了一个值，但“ RTs”列中将有更多RT值，而“ Key_Name”列中将有更多键名。