diff --git a/src/problem/SparseFEM.jl b/src/problem/SparseFEM.jl
index a9bb6a4f..3a1c98c8 100644
--- a/src/problem/SparseFEM.jl
+++ b/src/problem/SparseFEM.jl
@@ -20,7 +20,7 @@ end
 
 # timing of assembly, slice and solve
 function perf_sparse_fem(N)
-    Ifree = get_free(N)
+    Ifree = map(i -> LinearIndices((N, N))[i], get_free(N))
     # assembly
     A = fdlaplacian(N)
     # boundary condition