diff --git a/src/System/OnsiteCoupling.jl b/src/System/OnsiteCoupling.jl
index 9c617cd99..3f7c9d132 100644
--- a/src/System/OnsiteCoupling.jl
+++ b/src/System/OnsiteCoupling.jl
@@ -3,6 +3,11 @@ function onsite_coupling(sys, site, matrep::AbstractMatrix)
     size(matrep) == (N, N) || error("Invalid matrix size.")
     matrep ≈ matrep' || error("Operator is not Hermitian")
 
+    if N == 2 && isapprox(matrep, matrep[1, 1] * I; atol=1e-8)
+        suggest = sys.mode == :dipole ? " (use :dipole_large_s to reproduce legacy calculations)" : ""
+        @warn "Onsite coupling is always trivial for quantum spin s=1/2" * suggest
+    end
+
     if sys.mode == :SUN
         return Hermitian(matrep)
     elseif sys.mode == :dipole