Settings error when converting from .net 4 to .net 3.5

I change a settings file programmatically and save the changes, using Shapdevelop 4.3:

My_Settings.Default["BooleanProperty"] = false;

My_Settings.Default.Save();

It works fine as long as I used .NET 4.

When I convert the project to .NET 3.5 , I get the following error:

"An error occurred creating the confoguration section handler to MyApp.My_Settings. Could not load file or assembly for 'System Version = 4.0.0.0....' "

Any ideas?

Thanks in advance!