diff --git a/csrc/vertical_slash_index.cu b/csrc/vertical_slash_index.cu
index 45af042..7839343 100644
--- a/csrc/vertical_slash_index.cu
+++ b/csrc/vertical_slash_index.cu
@@ -60,12 +60,20 @@ __global__ void convert_vertical_slash_indexes_kernel(
 
     int tmp_col_cnt = 0, tmp_blk_cnt = 0;
     int s = 0, v = 0;
-    int v_idx = vertical_indexes[v++];
-    int s_idx = slash_indexes[s++];
-    while (s_idx >= end_m) {
+
+    // in case of vs are empty
+    int v_idx = (v < NNZ_V) ? vertical_indexes[v++] : (end_m + BLOCK_SIZE_M);
+    int s_idx = (s < NNZ_S) ? slash_indexes[s++] : -1;
+
+    // make sure s_idx is valid
+    while (s_idx >= end_m && s < NNZ_S) {
         s_idx = slash_indexes[s++];
     }
-    s_idx = max(end_m - s_idx, BLOCK_SIZE_M);
+    if (s_idx >= end_m) {
+        s_idx = end_m + BLOCK_SIZE_M;
+    } else {
+        s_idx = max(end_m - s_idx, BLOCK_SIZE_M);
+    }
     int range_start = s_idx - BLOCK_SIZE_M, range_end = s_idx;
     while (1) {
         if (v_idx < range_end) {