invalid argument pipe string is not function for pipe datepi

Question

i have error as below 

NbCalendarHeaderComponent.html:3 ERROR Error: InvalidPipeArgument: 'String is not a function' for pipe 'DatePipe'
    at invalidPipeArgumentError (common.js:6367)
    at DatePipe.transform (common.js:7594)
    at checkAndUpdatePureExpressionInline (core.js:43597)
    at checkAndUpdateNodeInline (core.js:44371)
    at checkAndUpdateNode (core.js:44306)
    at debugCheckAndUpdateNode (core.js:45328)
    at debugCheckRenderNodeFn (core.js:45306)
    at Object.eval [as updateRe

 

 

this error display when i click to select date from date picker 

 

error display and nothing select 

 

fromDateClick()
  {

   this.model.startdate = "";

  }

       <div class="col-sm">
          <label for="fromDate">FromDate</label>
          <input [nbDatepicker]="datepicker1" #startdate="ngModel" [(ngModel)]="model.startdate" name="startdate" class="form-control"
            (click)="fromDateClick()" type="text" readonly id="fromDate" placeholder="fromDate">
          <nb-datepicker #datepicker1></nb-datepicker>
        </div>

model : any={
    startdate :'',
    enddate:''

  };
so how to solve problem please ?