Employer stating I have to make up hours that they missed

Last year my employer made me aware that my previous shift pattern did not make up sufficient hours. They realised the mistake and informed me that as a result of this I now owed them 150 hours that I would now have to work back.

They had given me my shift pattern which I had worked for 18 months before being notified that the shift pattern they had given me didn’t have enough hours. It was their shift pattern that I was given when I started the particular role. I was unaware over the 18 months that the pattern was incorrect and my employer was also unaware for that period and then they rectified this upon realising. They have informed me with calculations provided that i worked 150 hours under what was required during that period.

My union challenged this but have conceded defeat and stated that i now have to work an extra 150 hours to make up the missed hours. I am paid a salary so have already been paid.

Is this right? I was working what I was told to work and now I owe a substantial amount of hours that I was unaware I was accumulating. This is clearly their error and not mine.