--- spack-src/src/c/ssht_sampling.h
+++ spack-src/src/c/ssht_sampling.h
@@ -89,7 +89,7 @@
  */
 static inline void ssht_sampling_ind2elm(int *el, int *m, int ind) {
 
-  *el = sqrt(ind);
+  *el = lrint(floor(sqrt(ind)));
   *m = ind - (*el)*(*el) - (*el);
 
 }