elementUI checkBox error Cannot read property 'length' of undefined solution

I encountered such an error when using el-checkbox

TypeError: Cannot read property 'length' of undefined
at (?ccbf:6452)
at (?efeb:4482)
at (?efeb:4587)
at [as isLimitDisabled] (?efeb:4839)
at (?ccbf:6455)
at (?efeb:4482)
at (?efeb:4587)
at [as isDisabled] (?efeb:4839)
at (?efeb:2104)
at Proxy.checkboxvue_type_template_id_d0387074_render (?ccbf:6161)

In template, mine is bound like this:

<!-- Multiple choices -->
<template>
  <el-checkbox-group v-model="examData[current].answer">
    <el-checkbox 
      :label="item"
      v-for="(item, index) in examData[current].tmDa"
      @change="examData[current].complete = true">
      <strong>{{('A'.charCodeAt(0) + index)}}</strong>
      <span>{{item}}</span>
    </el-checkbox>
  </el-checkbox-group>
</template>

In the data options:

data() {
  return {
    examData: [
      {
        answer: [],
        title: 'Question, question, question?  ',
        options: [
          'Answer 1',
          'Answer 2',
          'Answer 3',
          'Answer 4',
        ],
        complete: false,
      },
    ]
  }
}

Reason for the error:

In data, we are actually our static data. The data obtained from the backend is assigned to examData. I found that there is no answer field in examData

We have to add an answer field to the data and give the type to an array

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