matlab question with sym

When I using the function of 'norm' with norm(F,Inf),the command window shows a line of the words with "Undefined function or method 'norm' for input arguments of type 'sym'". Actually,F is a matrix with real number,however, it comes from some other matrix which is define by sym. So ,How I can make norm(F,Inf) successfully?

Answer 1

[Walter Roberson]

norm(double(F),Inf)