__NSCFConstantString objectForKey unrecognized selector sent to instance error

__NSCFConstantString objectForKey unrecognized selector sent to instance
error

I basically get this error
'NSInvalidArgumentException', reason: '-[__NSCFConstantString
objectForKey:]: unrecognized selector sent to instance 0x581f0'
on my program. I think it refers to this call I make,
if (data != nil) {
if([data objectForKey:@"username"]){
// NSArray *check= [[NSArray alloc]init];
//check=[data allValues];
[dict setObject:[data allValues] forKey:@"args"];
}else{
[dict setObject:[NSArray arrayWithObject:data] forKey:@"args"];
}
at the setObject:[data allValues]. I don't know why it gives that error
but data is an NSDictionary and I'm getting all the values and placing it
in an array.