Incorrect checksum for freed object when adding object to NSMutableArray.

Question

Why does this code:

QuadTree *q1 = [[QuadTree alloc] initWithCGRect:CGRectMake(m_bounds.origin.x, m_bounds.origin.y, halfWidth, halfHeight)];

[m_nodes addObject:q1];
[m_nodes addObject:[[QuadTree alloc] initWithCGRect:CGRectMake(m_bounds.origin.x, m_bounds.origin.y + halfHeight, halfWidth, halfHeight)]];
[m_nodes addObject:[[QuadTree alloc] initWithCGRect:CGRectMake(m_bounds.origin.x + halfWidth, m_bounds.origin.y, halfWidth, halfHeight)]];
[m_nodes addObject:[[QuadTree alloc] initWithCGRect:CGRectMake(m_bounds.origin.x + halfWidth, m_bounds.origin.y + halfHeight, halfWidth, halfHeight)]];

NSLog(@"nodes count %lu", (unsigned long)m_nodes.count);

Returns this message:

malloc: *** error for object 0xa852234: incorrect checksum for freed object - object was probably modified after being freed.
*** set a breakpoint in malloc_error_break to debug
nodes count 0

1. what does the incorrect checksum message means and how to solve it?

2. why does the array count is zero?

Notes: This is the declaration of m_nodes.

NSMutableArray *m_nodes;
m_nodes = [[NSMutableArray alloc] initWithCapacity:4];