iphone - Accessing global variable from within Switch statement -

i have property called total nsdecimalnumber. in switch statement i'm trying simple addition , subtraction on number keep getting 'unrecognized selector sent instance' error. here code:

switch (t) { case 0: {     if (_zero == yes)     {         _total = [_total decimalnumberbysubtracting:[nsdecimalnumber decimalnumberwithstring:@"4.5"]];         = (uiimageview *)[self.view viewwithtag:7];         i.hidden = yes;     } else {         nsdecimalnumber *fourpointfive = [[nsdecimalnumber alloc] initwithdecimal:[[nsnumber numberwithfloat:4.5] decimalvalue]];         _total = [_total decimalnumberbyadding:fourpointfive];         = (uiimageview *)[self.view viewwithtag:7];         i.hidden = no;     }     break; 

in same method, before switch statement initialize it:

_total = [[nsdecimalnumber alloc] initwithdecimal:[[nsnumber numberwithfloat:0] decimalvalue]]; 

and in @interface:

@property (strong, nonatomic) nsdecimalnumber *total; 

update:

changed initialization to:

_total = [[nsdecimalnumber alloc] initwithstring:@"0"]; 

but it's still crashing. removed other references decimalvalue.

this error message get:

terminating app due uncaught exception 'nsinvalidargumentexception', reason: '-[__nscfnumber decimalnumberbyadding:]: unrecognized selector sent instance 

on ios nsnumber's decimalvalue returns nsdecimal, not nsdecimalnumber.