matlab字符串符号型出错？

如图
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)

sym是变量，char是字符，可以直接['要加的内容']