complex, real, common ring, crash in 'vector' constructor

edit

(x,y,z) = var('x,y,z')
x = z.real_part()
y = z.imag_part()
test = vector((x,1,1))
test = vector((x,y,1))
test = vector((z.real_part,1,1))

Last line causes an error, can't find common ring for all elements.
These variables all have type "Expression"; but some expressions denote complex numbers and some denote real numbers. How do I tell sage about that? (Sage 6.5 on Mac OS 10).