  We present measurements of the inclusive branching fractions for the decays
D^+ -> X e^+ nu_e and D^0 -> X e^+ nu_e, using 281 pb^-1 of data collected on
the psi(3770) resonance with the CLEO-c detector. We find Br(D^0 ->Xe^+\nu_e) =
(6.46 \pm 0.17 \pm 0.13)% and Br((D^+ -> Xe^+nu_e) = (16.13 \pm 0.20 \pm
0.33)%. Using the known D meson lifetimes, we obtain the ratio
Gamma{D^+}^sl/Gamma_{D^0}^sl= 0.985\pm 0.028\pm 0.015, confirming isospin
invariance at the level of 3%. The positron momentum spectra from D^+ and D^0
have consistent shapes.
