error: "car: expects argument of type <pair>; given ()" on specific input

[zwizwa]

The following 2-line input to `parse-program' produces the error:
"car: expects argument of type <pair>; given ()"

void foo(void *x, void *y) { int i = foo(x, y); }
void bar(void *x, void *y) { int i = bar(x, y); }