Trouble extending NSMutableArray

I'm trying to write a category extension of NSMutableArray to load a
file which contains comma delimited lines of text. I have this:

// Initialize newly alloc'd array with a comma separated values file
- (id)initWithContentsOfCSVFile:(NSString *)path
{
if (self = [super init])
{
NSString * file = [path stringByExpandingTildeInPath];
NSString * data = [NSString stringWithContentsOfFile:file];
NSArray * list = [data componentsSeparatedByString:@"\r"];
int i;

for (i=0; i<[list count]; i++)
{
NSString * line = [list objectAtIndex:i];
NSMutableArray * csv = [[NSMutableArray alloc] initWithArray:
[line componentsSeparatedByString:@","]];

// Add the CSV array not the line as text to our array
[self addObject:csv];
[csv release];
}
}
return self;
}

and called like this:

NSMutableArray * array = [[NSMutableArray alloc]
initWithContentsOfCSVFile:@"~/sample.csv"];

but upon entering the method, it immediately get this exception:

<NSInvalidArgumentException> *** -count send to an uninitialized
mutable array object

If I change the caller to do the init first:

NSMutableArray * array = [[[NSMutableArray alloc] init]
initWithContentsOfCSVFile:@"~/sample.csv"];

I don't get this. In the method it says 'self' is a

NSPlaceholderMutableArray

and its super is a

NSMutableArray

so I wasn't sure if the 'init' being called should be [self init] or
[super init] but the immediate exception tells me I'm doing something
wrong, but what?

I had planned to return nil after releasing 'self' is anything was wrong
with the file after some initial checking, but it seems I'm stuck at
square 1. TIA

/los "I was a teenage net-random"