matlab convertChar报错

错误使用 sym>convertChar (line 1548)
Character vectors and strings in the first argument can only specify
a variable or number. To evaluate character vectors and strings
representing symbolic expressions, use 'str2sym'.

出错 sym>tomupad (line 1255)
        S = convertChar(x);

出错 sym (line 222)
                S.s = tomupad(x);

出错 Untitled3 (line 2)
f1=sym('exp(-0.5*t)*(heaviside(t)-heaviside(t-2))');
一直在报错 想知道这个是什么意思