Credit Question

I have no credit cards but Amex and pay off full balance at end of each month. I never have missed an auto payment and my house is paid off, no mortgage or HELOC. I went to lease a car and was told my credit was not high enough. Was told that not having debt can lower the credit rating! Is this accurate because it seems that one can only have a high credit rating if they are in debt and only paying the interest and never the principal.