Why does web form return fields to Python CGI as MiniFieldStorage?

In one python script I have used the following to create a web form to submit data:

print("<form action='newspeciescheck.py' method='post'>")
print("<p>Genus: <input  type='text' name='newgenus'/> <p>Species: <input type='text' name='newspecies'/>")
print("<input type='submit' value='Enter'>")
print("</form>\n ")

In the target script I have

formfields = cgi.FieldStorage()
newgenus = formfields.getValue('newgenus')
newspecies = formfields.getValue('newspecies')

However, when I try to run this, cgitb throws an error telling me that:

A problem occurred in a Python script. Here is the sequence of function calls leading up to the error, in the order they occurred.


C:\Abyss Web Server\htdocs\pyscripts\newspeciescheck.py in ()
     21 
     22 formfields = cgi.FieldStorage()
=>   23 newgenus = formfields.getValue('newgenus')
     24 status=''
     25 if newgenus==None:
newgenus undefined, formfields = FieldStorage(None, None, [MiniFieldStorage('newg...phis'), MiniFieldStorage('newspecies', 'fabae')]), formfields.getValue undefined
 C:\Users\John\AppData\Local\Programs\Python\Python36\lib\cgi.py in __getattr__(self=FieldStorage(None, None, [MiniFieldStorage('newg...phis'), MiniFieldStorage('newspecies', 'fabae')]), name='getValue')
    583     def __getattr__(self, name):
    584         if name != 'value':
=>  585             raise AttributeError(name)
    586         if self.file:
    587             self.file.seek(0)
builtin AttributeError = <class 'AttributeError'>, name = 'getValue'
AttributeError: getValue 
      args = ('getValue',) 
      with_traceback = <built-in method with_traceback of AttributeError object>

So why do my submitted values end up in MiniFieldStorage rather than normal FieldStorage?