When I destroy the weldJoint in the update method (see below) I get an EXC_BAD_ACCESS error pointing to the line

world->DestroyJoint(weldJoint);

in the update method below:

    -(void) update: (ccTime) dt
{

int32 velocityIterations = 8;
int32 positionIterations = 1;

// Instruct the world to perform a single step of simulation. It is
// generally best to keep the time step and iterations fixed.
world->Step(dt, velocityIterations, positionIterations);

// using the iterator pos over the set
std::set<BodyPair *>::iterator pos;

for(pos = bodiesForJoints.begin(); pos != bodiesForJoints.end(); ++pos)
{

    b2WeldJointDef weldJointDef;

    BodyPair *bodyPair = *pos;
    b2Body *bodyA = bodyPair->bodyA;
    b2Body *bodyB = bodyPair->bodyB;

    weldJointDef.Initialize(bodyA, bodyB, bodyA->GetWorldCenter());

    weldJointDef.collideConnected = false;
    weldJoint = (b2WeldJoint*) world->CreateJoint(&weldJointDef);

    // Free the structure we allocated earlier.
    free(bodyPair);

    // Remove the entry from the set.
    bodiesForJoints.erase(pos);
}

for(b2Body *b = world->GetBodyList(); b; b=b->GetNext())    {
    if (b->GetUserData() != NULL)
    {
        CCSprite *mainSprite = (CCSprite*)b->GetUserData();
        if (mainSprite.tag == 1) {
            mainSprite.position = CGPointMake( b->GetPosition().x * PTM_RATIO, b->GetPosition().y * PTM_RATIO);
            CGPoint mainSpritePosition = mainSprite.position;
            if (mainSprite.isMoved) {

                        world->DestroyJoint(weldJoint);

                    }
            }
        }
    }

}  

In the HelloWorldLayer.h I set the weldJoint with the assign property. Am I destroying the joint in the wrong way? I would really appreciate any help. Thanks