Unexpected nullability warning is reported for erroneous conditional assignment

[AlekseyTs]

        [Fact]
        public void Test()
        {
            string source = @"
#nullable enable

public class C
{
    public static void Main()
    {
        C2? c = null;
        var a = c ??= ""2"";
        Print(a);        
    }

    static void Print(dynamic b) 
    {
    }
}

class C2 {}
";

            var comp = CreateCompilation(source, options: TestOptions.DebugExe, targetFramework: TargetFramework.StandardAndCSharp);

            comp.VerifyDiagnostics(
                // (9,17): error CS0019: Operator '??=' cannot be applied to operands of type 'C2' and 'string'
                //         var a = c ??= "2";
                Diagnostic(ErrorCode.ERR_BadBinaryOps, @"c ??= ""2""").WithArguments("??=", "C2", "string").WithLocation(9, 17),
                // (9,23): warning CS8619: Nullability of reference types in value of type 'string' doesn't match target type 'C2'.
                //         var a = c ??= "2";
                Diagnostic(ErrorCode.WRN_NullabilityMismatchInAssignment, @"""2""").WithArguments("string", "C2").WithLocation(9, 23)
                );
        }

The nullability warning doesn't really make sense. The nullability actually matches, the types are different though.

[AlekseyTs]

CC @333fred