Fixed a discrepancy in `OP_ASET`

There was a discrepancy in the actual behavior, assertions, and documentation.
Therefore, I modified it based on the actual behavior.

1 files changed, 1 insertions, 1 deletions

diff --git a/src/vm.c b/src/vm.c
index 3457ce33c..f6b929ad6 100644
--- a/src/vm.c
+++ b/src/vm.c
@@ -2669,7 +2669,7 @@ RETRY_TRY_BLOCK:
     }
 
     CASE(OP_ASET, BBB) {
-      mrb_assert(mrb_type(regs[a]) == MRB_TT_ARRAY);
+      mrb_assert(mrb_type(regs[b]) == MRB_TT_ARRAY);
       mrb_ary_set(mrb, regs[b], c, regs[a]);
       NEXT;
     }