I am getting cast over flow . not able to under stand why this error is coming

code:

u16 b;
u16 a;

a=b+1;

a=(a>9)?(a-10):(a)

error:

cast_overflow : Truncation due to cast operation on (a>9)?a-10:a from 32 to 16 bits.
overflow_assign: a is assigned from (a>9)?(a-10):a.

all the variables are u16