Getter and @Nonnull

I get a warning from eclipse and I know I can remove it with suppress warning but I'd prefer to understand what makes it thing it could be null.

package-info.java

@ParametersAreNonnullByDefault
package test;

import javax.annotation.ParametersAreNonnullByDefault;

test.java

package test;


public class Test {
    public static void main( final String[ ] args ) {
        System.out.println( new Test( "a" ).getS( ) );
    }

    private final String s;

    public Test( final String s ) {
        this.s = s;
    }

    public String getS( ) {
        return this.s;//Null type safety: The expression of type String needs unchecked conversion to conform to '@Nonnull String'
    }
}

I don't get why I get this warning...

PS:

public Test( @Nonnull final String s ) { -> The nullness annotation is redundant with a default that applies to this location

@Nonnull private final String s; -> nothing changes