How to solve "The argument type 'List<String>' can't be assigned to the parameter type 'String"

Here is my code

final _months = [
    ["January", "Febuary", "March", "April", "May", "June", "July", "August", "September", "October", "November", "December"]
  ];

......

DropdownButtonFormField(
              hint: const Text('Select Period'),
              items: _months.map((e) {
                return DropdownMenuItem(
                  value: e,                                                      
                  child: Text(e //here is the error showing),
                );
              }).toList(),
              onChanged: (value) {
                //do something
              },
            ),

How to solve this , i just changed the months map value to dynamic from dfault, then the error is gone, but when i run the app it showing the another error like "this is not subtype of string"
how to fix this and why is this happening

>Solution :

You have an extra set of [ ] so your _months variable is of type List<List<String>> :

  final _months = ["January", "Febuary", "March", "April", "May", "June", "July", "August", "September", "October", "November", "December"];