diff --git a/loopy/transform/subst.py b/loopy/transform/subst.py
index 79ceff9fdf1e2c4b3b544e8ae85f8194b36ec444..a681afe06520483c83530c241e39229412e88f03 100644
--- a/loopy/transform/subst.py
+++ b/loopy/transform/subst.py
@@ -1,6 +1,4 @@
-from __future__ import division
-from __future__ import absolute_import
-import six
+from __future__ import division, absolute_import
 
 __copyright__ = "Copyright (C) 2012 Andreas Kloeckner"
 
@@ -24,6 +22,7 @@ OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN
 THE SOFTWARE.
 """
 
+import six
 
 from loopy.symbolic import (
         get_dependencies, SubstitutionMapper,
@@ -141,6 +140,7 @@ def extract_subst(kernel, subst_name, template, parameters=()):
     dfmapper = CallbackMapper(gather_exprs, WalkMapper())
 
     for insn in kernel.instructions:
+        dfmapper(insn.assignees)
         dfmapper(insn.expression)
 
     for sr in six.itervalues(kernel.substitutions):
@@ -178,8 +178,7 @@ def extract_subst(kernel, subst_name, template, parameters=()):
     new_insns = []
 
     for insn in kernel.instructions:
-        new_expr = cbmapper(insn.expression)
-        new_insns.append(insn.copy(expression=new_expr))
+        new_insns.append(insn.with_transformed_expressions(cbmapper))
 
     from loopy.kernel.data import SubstitutionRule
     new_substs = {
diff --git a/test/test_loopy.py b/test/test_loopy.py
index f2658ca75fe726a809ea7e30b1263e51b93c1716..d0398f216a7f85798bc5f125e353578e74765b9f 100644
--- a/test/test_loopy.py
+++ b/test/test_loopy.py
@@ -2678,6 +2678,30 @@ def test_preamble_with_separate_temporaries(ctx_factory):
         queue, data=data.flatten('C'))[1][0], data[offsets[:-1] + 1])
 
 
+def test_add_prefetch_works_in_lhs_index():
+    knl = lp.make_kernel(
+            "{ [n,k,l,k1,l1,k2,l2]: "
+            "start<=n<end and 0<=k,k1,k2<3 and 0<=l,l1,l2<2 }",
+            """
+            for n
+                <> a1_tmp[k,l] = a1[a1_map[n, k],l]
+                a1_tmp[k1,l1] = a1_tmp[k1,l1] + 1
+                a1_out[a1_map[n,k2], l2] = a1_tmp[k2,l2]
+            end
+            """,
+            [
+                lp.GlobalArg("a1,a1_out", None, "ndofs,2"),
+                lp.GlobalArg("a1_map", None, "nelements,3"),
+                "..."
+            ])
+
+    knl = lp.add_prefetch(knl, "a1_map", "k")
+
+    from loopy.symbolic import get_dependencies
+    for insn in knl.instructions:
+        assert "a1_map" not in get_dependencies(insn.assignees)
+
+
 if __name__ == "__main__":
     if len(sys.argv) > 1:
         exec(sys.argv[1])