diff --git a/grudge/symbolic/mappers/__init__.py b/grudge/symbolic/mappers/__init__.py
index d7f3e71c567821f164574d33cc671203526ddcf0..745d56be2a0604bc64b26c7d3add125131835480 100644
--- a/grudge/symbolic/mappers/__init__.py
+++ b/grudge/symbolic/mappers/__init__.py
@@ -524,7 +524,7 @@ class GlobalToReferenceMapper(CSECachingMapperMixin, IdentityMapper):
                 1/jac_in * self.rec(expr.field))
 
         elif isinstance(expr.op, op.FaceMassOperator):
-            jac_in_surf = sym.area_element(self.ambient_dim, self.dim - 1,
+            jac_in_surf = - sym.area_element(self.ambient_dim, self.dim - 1,
                     dd=expr.op.dd_in)
             return op.RefFaceMassOperator(expr.op.dd_in, expr.op.dd_out)(
                     jac_in_surf * self.rec(expr.field))