a working program Simulink and Stateflow chart in 2012a cannot be run when re-typed into 2013a

A stateflow chart has u (a continuous output from simulink block) and b2 (a constant) as inputs. In the chart a Matlab function uses b2 and u to generate an output Phi.

function Phi = Phi_1(b2) Phi=sin(pi*u/b2)*sin(pi*u/b2);

The Matlab function lists b2 as input and Phi as output

When the program is run, the following error is posted:

Symbol 'b2' could resolve to the data 'b2' (#46), or to the data 'b2' (#55) of the same name. Please rename one of these objects.

Thanks for the help. Tony