Gryphel is right, and bbraun helped me figure it out. I am using Codewarrior 10, which automatically slips calls to _StripAddress into the generated code. That call isn't necessary on a Mac Plus, since it's always in 24 bit mode, but since _StripAddress isn't implemented in the Plus ROM nor in System 3.2, the program bombs out. There doesn't seem to be any Codewarrior option to disable this "helpful" behavior, but using ResEdit I was able to search the generated code for the _StripAddress trap (0xA055) and replace it with a NOP (0x4E71). Now everything is happy.