why can't I contribute to an IRA when my wife has a SEP-IRA (but I don't)?

My wife is contributing to her SEP-IRA but I do not have one (I do have a schedule C, but I am not making a SEP contribution from it).  I'd like to contribute to a Traditional IRA instead.  But TT tells me our income is too high while being covered by a retirement plan at work.  I get that my wife's SEP means she's covered by a retirement plan at work, but why is that affecting my contribution?   Shouldn't the two be separate?