I had a similar error. In my case, the problem was that I was trying to alter an existing table by adding a foreign key reference to a table that had existing data in it. I had a previous migration that added some test data for me. Specifically, for me, the problem was that I was trying to explicitly add the constraint in MySQL. I removed the constraint and let rails handle the relationship (probably not the best idea, but I am still learning and playing) and the error went away.