Differentiate - x data does not match y data?

I'm having a bit of an issue here. When I try to use Differentiate with a specified x wave, I keep getting "X data does not match Y data (length or number type)." However, when I check to make sure that the types and lengths are the same, I get

print numpnts(energy), numpnts(t_1_r)
  3255  3255
print numtype(energy(1)), numtype(t_1_r(1))
  0  0


So I'm not sure what the problem could be. The command I'm using is
Differentiate t_1_r/X=Energy/D=t_1_r_dif
You should use the WaveType() function to make sure that the source and X wave types are the same (or use the Data Browser), not numtype. Your two numtype statements are just telling you that the value in your two waves at x=1 is a normal number, but that doesn't tell you anything about the wave type.