Making a character variable equal to a nill value in delphi

Making a character variable equal to a nill value in delphi

I am creating a programme where I am searching for vowels within a string,
and after they have been found, the vowels need to be removed from the
string.
mystring := 'August' ;
{if mystring[1] IN ['A','E','I','O','U'] ;}
for k := 1 to length(mystring) DO
BEGIN
if mystring[K] IN ['a','e','i','o','u']
then mystring[k] := ''
but the error
Incompatible types: 'Char' and 'String' " appears
What can i do to delete the vowels from my string?