-[__nscfnumber Rangeofcharacterfromset:]: Unrecognized selector sent to instance 0x7fa5216589d0

This kind of mismatch, that is, the NSNumber type of the assignment to the string itself is not known, because my test, the code is as follows (VIEWCONTROLLER.M)

-(void) Viewdidload {
[Super Viewdidload];
BankAccount = [[Bankobject alloc] init];
[BankAccount setvalue:@ "ten" forkey:@ "account"];
[BankAccount addobserver:self forkeypath:@ "account" options:nskeyvalueobservingoptionnew| Nskeyvalueobservingoptionold Context:nil];

_mylabel = [[UILabel alloc] initwithframe:cgrectmake (100, 100, 200, 100)];
_mylabel.text = [BankAccount valueforkey:@ "account"];
_mylabel.textcolor = [Uicolor Whitecolor];
[Self.view Addsubview:_mylabel];

_mybutton = [UIButton buttonwithtype:uibuttontyperoundedrect];
_mybutton.frame = CGRectMake (200, 300, 100, 50);
_mybutton.backgroundcolor = [Uicolor Redcolor];
[_mybutton addtarget:self Action: @selector (ChangeAccount) forcontrolevents:uicontroleventtouchupinside];
[Self.view Addsubview:_mybutton];

}

Break point, is also the program after the execution of Viewdidload will not collapse, unaware of _mylabel.text = [BankAccount valueforkey:@ "Account"]; This error, changed to _mylabel.text = [ NSString stringwithformat:@ "%@", [BankAccount valueforkey:@ "Account"]; it's OK.

-[__nscfnumber Rangeofcharacterfromset:]: Unrecognized selector sent to instance 0x7fa5216589d0