Who/Whom question - grammar nits only please

[maryfield48]

My wife, who teaches prep school (grade school, I guess, to you Yanks), came across a set of who/whom questions on a web site for teachers. This is one of them:

"(Who/Whom) did you say was calling?"

The site gives Whom as the answer. Neither she nor I can figure this out - both of us, along with everyone else she has asked, think it should be Who. Are we missing something or is the web site?