The error message "Cannot read properties of undefined (reading 'length')" occurred in the Vue Element UI checkbox component

When using the el-checkbox component from Element UI, i encountered the error message "Cannot read properties of undefined (reading 'length')", and the checkbox is not being displayed. These are my codes：

... return{ ... ruleForm: { name: '', code: '', phone: '', sex: '', ethinc: '', date1: '', date2: '', desc: '', course: [], }, ... }

I tried to move the course array outside of ruleForm, but the validator don't work