diff --git a/awkward-cpp/src/cpu-kernels/awkward_Index_nones_as_index.cpp b/awkward-cpp/src/cpu-kernels/awkward_Index_nones_as_index.cpp
index bef9e2b83d..ad2febc39e 100644
--- a/awkward-cpp/src/cpu-kernels/awkward_Index_nones_as_index.cpp
+++ b/awkward-cpp/src/cpu-kernels/awkward_Index_nones_as_index.cpp
@@ -9,14 +9,14 @@ ERROR awkward_Index_nones_as_index(
   T* toindex,
   int64_t length) {
   int64_t n_non_null = 0;
-  // Assuming that `toindex` comprises of contiguous integers, and is zero-based
+  // Assuming that `toindex` comprises of unique, contiguous integers (or -1), and is zero-based
   // Compute the number of non-null values to determine our starting index
   for (int64_t i = 0; i < length; i++) {
     if (toindex[i] != -1) {
         n_non_null++;
     }
   }
-  //
+  // Now set the null-value indices to by monotonically increasing and unique from the final index
   for (int64_t i = 0; i < length; i++) {
     toindex[i] == -1 ? toindex[i] = n_non_null++ : toindex[i];
   }