diff --git a/python/dune/perftool/loopy/transformer.py b/python/dune/perftool/loopy/transformer.py
index 3ad304f1310683d17fbc62c312cebeba6e5e5c4b..71681e958ff46a9121e0b0aee1159bbd18c9057b 100644
--- a/python/dune/perftool/loopy/transformer.py
+++ b/python/dune/perftool/loopy/transformer.py
@@ -53,11 +53,25 @@ class UFL2LoopyVisitor(ModifiedTerminalTracker, UFL2PymbolicMapper):
         super(UFL2LoopyVisitor, self).__init__()
 
     def coefficient(self, o):
-        # All trial functions should already be handled by the TrialFunctionExtractor class!
+        # All trial functions should already be handled
         assert o.count() != 0
         # TODO implement non-trialfunction coefficients
         raise NotImplementedError
 
+    def index_sum(self, o):
+        from dune.perftool.ufl.shape import determine_shape
+
+        # Define an iname for each of the indices in the multiindex
+        for i in o.ufl_operands[1].indices():
+            shape = determine_shape(o.ufl_operands[0], i)
+            from dune.perftool.pdelab import name_index
+            name = name_index(i)
+            iname(name)
+            domain(name, shape)
+
+        # Now continue processing the expression
+        return self.call(o.ufl_operands[0])
+
     # TODO use multiple inheritance and have a geometry transformer in the pdelab subpackage
     def facet_area(self, o):
         from pymbolic.primitives import Variable