Convenience initializer not working: "unrecognized selector"

FROM : K. Darcy Otto
DATE : Thu Mar 13 04:25:48 2008

I'm trying to create an initializer that allows me to combine 
initialization and some setting of variables, but I get an error that 
an "unrecognized selector" has been sent to the object. Here is what I 
have so far:

Fraction.h:
#import <Cocoa/Cocoa.h>

@interface Fraction : NSObject {

   NSInteger numerator;
   NSInteger denominator;
   
}

@property(readwrite) NSInteger numerator;
@property(readwrite) NSInteger denominator;

-(id)initWith:(NSInteger)aNumerator over:(NSInteger)aDenominator;

@end

Fraction.m:
#import "Fraction.h"

@implementation Fraction

@synthesize numerator,denominator;

-(id)init
{
   return [self initWith:1 over:1];
}

-(id)initWith:(NSInteger)aNumerator over:(NSInteger)aDenominator
{
   if (self = [super init]){
       self.numerator = aNumerator;
       self.denominator = aDenominator;
   }
   return self;
}

@end

Now, I can do the following with no ill effects:
Fraction *frac = [[Fraction alloc] init];
frac.numerator = 2;
frac.denominator = 2;

But if I do this, I get the unrecognized selector error:
Fraction *frac1 = [Fraction initWith:2 over:2];

Can anyone explain the error or suggest a solution? Thanks.