Parser Error Message: This method cannot be called during the application's pre-start initialization stage.

In a Web Site I'm getting this error:
 
Parser Error Message: This method cannot be called during the application's pre-start initialization stage.
 
This is my web.config file. I basically copied it from a WebForms project where it works just fine. The error is on the line that adds the membership provider.
 
<?xml version="1.0"?> <configuration>   <connectionStrings> ...