diff --git a/src/svd_trunc.jl b/src/svd_trunc.jl
index ebfee14..87a7e53 100644
--- a/src/svd_trunc.jl
+++ b/src/svd_trunc.jl
@@ -37,7 +37,7 @@ function (svd_trunc::TruncThresh)(M::AbstractMatrix)
     U, λ, V = svd(M)
     λ_norm = norm(λ)
     mprime = 1
-    s = 0.0
+    s = last(λ)^2
     for (k, λₖ) in Iterators.drop(Iterators.reverse(pairs(λ)), 1)
         s += λₖ ^ 2
         if s ≥ (λ_norm * svd_trunc.ε)^2
@@ -148,4 +148,4 @@ summary(svd_trunc::TruncBondThresh) = "SVD truncation with truncation to bond si
 
 function summary_compact(svd_trunc::TruncBondThresh) 
     ("SVD tolerance, m'", string(svd_trunc.ε)*", "*string(svd_trunc.mprime))
-end
\ No newline at end of file
+end