Using ln function with symbolic variables

I am using symbolic variables, and I would like to use an ln function. But I keep getting an error. Is it possible at all to involve a logarithmic function with symbolic variables?

I have

syms x1 x2
f=ln(x1+x2);

and I am gettingthe error: Undefined function 'ln' for input arguments of type 'sym'.

[Del]

I just tried log instead of ln, and it is working