This error occurs when expression the ngModel directive is bound to is a non-assignable expression.
Examples using assignable expressions include:
<input ng-model="namedVariable">
<input ng-model="myObj.someProperty">
<input ng-model="indexedArray[0]">
Examples of non-assignable expressions include:
<input ng-model="foo + bar">
<input ng-model="42">
<input ng-model="'oops'">
<input ng-model="myFunc()">
Always make sure that the expression bound via ngModel directive can be assigned to.
For more information, see the ngModel API doc.