Raspberry Preserves

[Chillytwinkle]

Hi, I have a question about Raspberry preserves.
I've never made preserves with bought frozen raspberries.
Well, I bought the frozen raspberries, and made the preserves, but when
we tasted the preserves, it tasted like sesame seeds.
I'm not sure why.
Have any of you had this happen?
And what could cause it to taste like that. I didn't do anything different than I usually do when making them.
It was the strangest thing.
thanks