diff --git a/loopy/kernel/creation.py b/loopy/kernel/creation.py
index f60662707962100cdba58a63b0295d215a76ffe5..c13fd295baaaf93c306466d84b946a5275a6103a 100644
--- a/loopy/kernel/creation.py
+++ b/loopy/kernel/creation.py
@@ -1038,7 +1038,13 @@ class CSEToAssignmentMapper(IdentityMapper):
         self.add_assignment = add_assignment
         self.expr_to_var = {}
 
-    def map_common_subexpression(self, expr):
+    def map_reduction(self, expr, additional_inames):
+        additional_inames = additional_inames | frozenset(expr.inames)
+
+        return super(CSEToAssignmentMapper, self).map_reduction(
+                expr, additional_inames)
+
+    def map_common_subexpression(self, expr, additional_inames):
         try:
             return self.expr_to_var[expr.child]
         except KeyError:
@@ -1048,19 +1054,20 @@ class CSEToAssignmentMapper(IdentityMapper):
             else:
                 dtype = None
 
-            child = self.rec(expr.child)
+            child = self.rec(expr.child, additional_inames)
             from pymbolic.primitives import Variable
             if isinstance(child, Variable):
                 return child
 
-            var_name = self.add_assignment(expr.prefix, child, dtype)
+            var_name = self.add_assignment(
+                    expr.prefix, child, dtype, additional_inames)
             var = Variable(var_name)
             self.expr_to_var[expr.child] = var
             return var
 
 
 def expand_cses(instructions, inames_to_dup, cse_prefix="cse_expr"):
-    def add_assignment(base_name, expr, dtype):
+    def add_assignment(base_name, expr, dtype, additional_inames):
         if base_name is None:
             base_name = "var"
 
@@ -1085,11 +1092,17 @@ def expand_cses(instructions, inames_to_dup, cse_prefix="cse_expr"):
                 assignee=Variable(new_var_name),
                 expression=expr,
                 predicates=insn.predicates,
-                within_inames=insn.within_inames,
+                within_inames=insn.within_inames | additional_inames,
                 within_inames_is_final=insn.within_inames_is_final,
                 )
         newly_created_insn_ids.add(new_insn.id)
         new_insns.append(new_insn)
+        if insn_inames_to_dup:
+            raise LoopyError("in-line iname duplication not allowed in "
+                    "an instruction containing a tagged common "
+                    "subexpression (found in instruction '%s')"
+                    % insn)
+
         new_inames_to_dup.append(insn_inames_to_dup)
 
         return new_var_name
@@ -1107,7 +1120,8 @@ def expand_cses(instructions, inames_to_dup, cse_prefix="cse_expr"):
 
     for insn, insn_inames_to_dup in zip(instructions, inames_to_dup):
         if isinstance(insn, MultiAssignmentBase):
-            new_insns.append(insn.copy(expression=cseam(insn.expression)))
+            new_insns.append(insn.copy(
+                expression=cseam(insn.expression, frozenset())))
             new_inames_to_dup.append(insn_inames_to_dup)
         else:
             new_insns.append(insn)