diff --git a/src/mps.jl b/src/mps.jl
index de71318..f5ca80e 100644
--- a/src/mps.jl
+++ b/src/mps.jl
@@ -896,7 +896,7 @@ function correlation_matrix(
             Li21 *= oᵢ * dag(psi[pL21])'
           else
             sᵢ = siteind(psi, pL21)
-            Li21 *= prime(dag(si[pL21]), !sᵢ)
+            Li21 *= prime(dag(psi[pL21]), !sᵢ)
           end
           Li21 *= psi[pL21]
         end
diff --git a/test/base/test_mps.jl b/test/base/test_mps.jl
index 65eed0e..015c06d 100644
--- a/test/base/test_mps.jl
+++ b/test/base/test_mps.jl
@@ -962,6 +962,9 @@ end
       @test inner(psi', MPO(a, s), psi) ≈ Cpm[i - ss + 1, j - ss + 1]
     end
 
+    # Test sites argument (also regression test for issue #105)
+    Cxz = correlation_matrix(psi, "X", "Z"; sites=2:2:6)
+
     # Electron case
     s = siteinds("Electron", 8)
     psi = random_mps(s; linkdims=m)