class_getInstanceMethod returns NULL

RVRenderHandler+PPSlowTransition.m

@implementation RVRenderHandler (PPSlowTransition)

+ (void) initialize
{
    Method method = nil;
    
    method = class_getInstanceMethod([self class], @selector(setTransitionTime:));
    method->method_name = @selector(_original_setTransitionTime:);
    
    // the above works
    
    method = class_getInstanceMethod([self class], @selector(_ppst_setTransitionTime:)); // returns NULL
    method->method_name = @selector(setTransitionTime:);
}

- (void) _ppst_setTransitionTime:(float)time
{
    NSLog(@"%f", time);
    [self _original_setTransitionTime:time];
}

@end