Stored procedure return value on multiple result sets

I have a procedure that validates certain data and in case the validation succeeds, it outputs 3 result sets and returns 0. If validation fails, then it just returns an error code (and error message in an output parameter).
In my C# code I am trying to check the return value before reading any of the possibly returned record sets. If my procedure successfully passes validation and returns the mentioned sets, I get System.ApplicationException : Attempting to cast a DBNull to a non nullable type! when I try to check return value before reading all the result sets. I get proper value if I first read all result sets.

Example:

using (var connection = new SqlConnection(GetConnectionString())
{
   var p = new DynamicParameters();
   p.Add("a", 11);
   p.Add("o", dbType: DbType.StringFixedLength, size: 500, direction: ParameterDirection.Output);
   p.Add("r", dbType: DbType.Int32, direction: ParameterDirection.ReturnValue);
   connection.Open();
   connection.Execute(@"DROP PROCEDURE IF EXISTS spEcho");
   connection.Execute(@"
CREATE PROCEDURE spEcho
    @a INT,
    @o NVARCHAR(500) OUT
AS 
BEGIN
    IF @a = 11
    BEGIN
        SELECT @a Id, 'ping' Name, 1 Id, 'pong1' Name
        SELECT @a Id, 'ping' Name, 2 Id, 'pong2' Name
    END
    ELSE
        SET @o = 'No data returned'
    RETURN @a
END");
   var grid = connection.QueryMultiple("spEcho", p,
                                       commandType: CommandType.StoredProcedure);
   int procRetVal = 0;
   //procRetVal = p.Get<int>("r"); //this statement fails with System.ApplicationException
   var result1 = grid.Read();
   var result2 = grid.Read();
   procRetVal  = p.Get<int>("r"); //this returns expected value
}

If validation fails, return value is retrieved properly.
I know I could alter the procedure and instead of using return value, I could return this information in additional result set, but this feels wrong to me. Is there any other way I can get store procedure's return value without going through all returned results?

Start by reproducing the supplied QueryMultiple call and DynamicParameters return-value registration, then inspect how consuming multiple result sets affects output and return parameters. Test both the two-result-set success branch and the no-result-set failure branch; done means establishing the supported retrieval behavior and documenting it or adding focused regression coverage.