diff --git a/ext/TensorKitCUDAExt/truncation.jl b/ext/TensorKitCUDAExt/truncation.jl
index 2633b3483..a4ee0009d 100644
--- a/ext/TensorKitCUDAExt/truncation.jl
+++ b/ext/TensorKitCUDAExt/truncation.jl
@@ -74,3 +74,11 @@ function MatrixAlgebraKit._ind_intersect(A::CuVector{Bool}, B::CuVector{Int})
     result[B] .= @view A[B]
     return result
 end
+
+function MatrixAlgebraKit.truncation_error!(values::CuSectorVector, ind::AbstractVector{Bool})
+    for (c, ind_c) in pairs(ind)
+        sector_vals = values[c]
+        @. sector_vals *= !ind_c
+    end
+    return norm(values)
+end