See title. I built a Frozen Heart on Amumu in a game, and then built a Chain Vest with the intention of building a Warden's Mail into a Randuin's Omen. Frozen Heart now builds out of a Warden's Mail (although, does not share the Warden's Mail passive). I did not get to complete the Randuin's Omen right out, but I was unable to purchase a Warden's Mail either by double-clicking or by clicking the "Buy" button.
1. Purchase a Frozen Heart.
2. Purchase a Chain Mail.
3. Attempt to purchase Warden's Mail.