diff --git a/src/quick_select.c b/src/quick_select.c
index 4c2318c..29fda45 100644
--- a/src/quick_select.c
+++ b/src/quick_select.c
@@ -46,7 +46,7 @@ double percentile(const double *arr, size_t n, double perc) {
   }
 
   double *arr_copy = malloc(sizeof(double)*n);
-  memcpy(arr_copy, arr, n);
+  memcpy(arr_copy, arr, n*sizeof(double));
   // nearest-rank: ceil(0.9*n) - 1
   size_t k = (size_t)(perc*n) - 1;
   double ans = select_k(arr_copy, n, k);