The reapers will come, but they fill not their sheaves from these. For the reapers will come, and will gather the wheat into the barn, and will bind the tares together, and cast them into the fire. Thus also is the grass of the house tops cleared off, and whatever is plucked from it, is thrown into the fire; because it had withered even before it was plucked up. The reaper fills not his hands thence. His next words are, "Whereof the reaper fills not his hand; neither he that binds up the sheaves his bosom" [Psalm 129:7]. And, "the reapers are the angels," [Matthew 13:39] the Lord says.
Source: Exposition on Psalm 129