When entering my T-slips, I try to put 0% for "your percentage" but it doesn't save - it reverts to 100%. Minimum I can put is 0.01%. How do I put 0%?

When entering my T-slips, I'm asked about "your percentage" (as compared to my spouses). I noticed when I put 0% and save, it doesn't work correctly - it reverts to 100% again when I check it. The minimum that saves correctly is 0.01%. Is this a bug? How do I enter 0%?