[en] We present numerical calculations of the nuclear spin-lattice relaxation (R_s) rate in the superconducting state of pure indium and slightly underdoped n-type cuprate Pr_0.91LaCe_0.09CuO_4-y. By properly taking into account electron-phonon coupling, our calculated R_s for the conventional s-wave superconductor, indium, is in quantitative agreement the experimental data with a clear Hebel-Slichter peak. In contrast, the absence of the Hebel-Slichter peak in the R_s data of Pr_0.91LaCe_0.09CuO_4-y can be explained by either d-wave or highly anisotropic s-wave gap symmetry. Thus, the absence of the Hebel-Slichter peak does not necessarily argue against an s-wave gap symmetry in this electron-doped cuprate. -- Highlights: → Nuclear spin-lattice relaxation rate in electron-doped Pr_0.91LaCe_0.09CuO_4-y. → The absence of the Hebel-Slichter peak agrees with either d-wave or anisotropic s-wave. → The gap symmetry in slightly underdoped n-type cuprates is of highly anisotropic s-wave.