diff --git a/loopy/transform/arithmetic.py b/loopy/transform/arithmetic.py
index 2939a74cfc921d27396974ce74aaeaab220cad43..181f5241137cc1abe3256506f3ebd59327e4c4f8 100644
--- a/loopy/transform/arithmetic.py
+++ b/loopy/transform/arithmetic.py
@@ -235,7 +235,7 @@ def collect_common_factors_on_increment(kernel, var_name, vary_by_axes=()):
             new_insns.append(insn)
             continue
 
-        index_key = insn.assignee.subscript
+        index_key = extract_index_key(insn.assignee)
 
         lhs = insn.assignee
         rhs = insn.expression