diff --git a/python/dune/perftool/sumfact/amatrix.py b/python/dune/perftool/sumfact/amatrix.py
index 6cadb6193d8c22b954e0db80825b5ad1897198e4..e3efb78fb93b0336dc89e9869dc4a83f9e47508b 100644
--- a/python/dune/perftool/sumfact/amatrix.py
+++ b/python/dune/perftool/sumfact/amatrix.py
@@ -72,7 +72,7 @@ class LargeAMatrix(ImmutableRecord):
     def name(self):
         name = "ThetaLarge{}{}_{}_qp{}".format("face{}_".format(self.face) if self.face is not None else "",
                                                "T" if self.transpose else "",
-                                               "_".join(tuple("d" if d else "" for d in self.derivative)),
+                                               "x".join(tuple("d" if d else "" for d in self.derivative)),
                                                quadrature_points_per_direction(),
                                                )
         for i, d in enumerate(self.derivative):