No loan lock until after appraisal?

I met with a mortgage broker who told me that his company doesn't allow him to lock loan rates until after the property appraisal. He also couldn't say with any certainty when the appraisal would occur. Is this normal?? With rates as low as they are I'm very concerned that waiting until after the appraisal to lock my rate will cause me to miss out on getting these incredibly low rates. I mean, no one can say for sure what the rates will be in 2 weeks, and if it's 2 weeks before the appraisal is done I'm just out of luck? It doesn't sound right to me but I wanted to get some other (hopefully) experienced opinions. Thanks.