(iphone) KERN_PROTECTION_FAILURE at ob_msgSend() - iphone

Question

I'm fairly certain KER_PROTECTION_FAILURE is happening due to the following code.
(I don't get the error when I skip calling this function)
What I suspect is that the following code is wrong? (data can be gone before string object is released or something like that)
NSMutableData* data = [NSMutableData dataWithLength: dataLength];
// populate data
return [[[NSString alloc] initWithData:data encoding:NSASCIIStringEncoding] autorelease];
Register $r0 shows the address of the returning NSString object.
and $r1 shows "release".
- (NSString *)encode:(const uint8_t *)input length:(NSInteger)length {
static char table[] = "ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789+/=";
int dataLength = ((length+2)/3)*4;
int remainder = dataLength % 16;
dataLength = dataLength + (16- remainder);
NSMutableData *data = [NSMutableData dataWithLength:
dataLength];
uint8_t *output = (uint8_t *)data.mutableBytes;
for (NSInteger i = 0; i < length; i += 3) {
NSInteger value = 0;
for (NSInteger j = i; j < (i + 3); j++) {
value <<= 8;
if (j < length) {
value |= (0xFF & input[j]);
}
}
NSInteger index = (i / 3) * 4;
output[index + 0] = table[(value >> 18) & 0x3F];
output[index + 1] = table[(value >> 12) & 0x3F];
output[index + 2] = (i + 1) < length ? table[(value >> 6) & 0x3F] :
'=';
output[index + 3] = (i + 2) < length ? table[(value >> 0) & 0x3F] :
'=';
}
return [[[NSString alloc] initWithData:data
encoding:NSASCIIStringEncoding]
autorelease]; }