GPUImageVideoCamera stops capturing after some frames when it has just one filter

[lusnaow]

videoCamera = [[GPUImageVideoCamera alloc] initWithSessionPreset:AVCaptureSessionPreset640x480 cameraPosition:AVCaptureDevicePositionBack];    
GPUImageFilter* em = [[GPUImageFilter alloc] init];    
[videoCamera addTarget:em];  
[videoCamera startCameraCapture];

Code above will stop capturing automatically after some frames.

videoCamera = [[GPUImageVideoCamera alloc] initWithSessionPreset:AVCaptureSessionPreset640x480 cameraPosition:AVCaptureDevicePositionBack];
GPUImageFilter* em = [[GPUImageFilter alloc] init];
[videoCamera addTarget:em];

GPUImageFilter* em2 = [[GPUImageFilter alloc] init];
[em addTarget:em2];

[videoCamera startCameraCapture];    

With two filters, works fine.