diff --git a/src/tensors/abstractblocktensor/sparsity.jl b/src/tensors/abstractblocktensor/sparsity.jl
index b0d3160..2d5ca83 100644
--- a/src/tensors/abstractblocktensor/sparsity.jl
+++ b/src/tensors/abstractblocktensor/sparsity.jl
@@ -6,7 +6,7 @@ nonzero_values(t::AbstractBlockTensorMap) = nonzero_values(parent(t))
 nonzero_length(t::AbstractBlockTensorMap) = nonzero_length(parent(t))
 
 nonzero_values(A::AbstractArray) = values(A)
-nonzero_keys(A::AbstractArray) = keys(A)
+nonzero_keys(A::AbstractArray) = eachindex(IndexCartesian(), A)
 nonzero_pairs(A::AbstractArray) = pairs(A)
 nonzero_length(A::AbstractArray) = length(A)