How to fill CHAR datatype in an MBO in SUP 2.1.3?

I use an MBO that has some attributes of type CHAR. In my generated iOS Code this is converted to type unichar.

When accessing the field in iOS I use the following code:

// setting the value
mbo.myfield = 'X';
// getting the value
NSString *myvalue = [NSString stringWithFormat:@%"C", mbo.myfield];

While using this in my app it seems to work as expected here. But when I try to save and submitPending I cannot see any changes in the cache db on SUP server, the value still is (null) for the field.

Do you have an idea how to use unichar fields in iOS? Maybe I have to set the value using characterAtString?