PI current controller problem in SIMULINK

hi all, I am using PI controller block in average current control mode of buck-boost converter. I need a sinusoidal current waveform with period 20ms. But the actual current output goes to zero 0.0005s before 1ms of first half cycle, similarly it comes to zero 0.0005s before 2ms. what is the problem? how to rectify it? please give me some suggestions....