diff --git a/NDTensors/src/blocksparse/diagblocksparse.jl b/NDTensors/src/blocksparse/diagblocksparse.jl
index 76cb29e566..a68b7a769c 100644
--- a/NDTensors/src/blocksparse/diagblocksparse.jl
+++ b/NDTensors/src/blocksparse/diagblocksparse.jl
@@ -639,6 +639,11 @@ function contract!(
   labelsT2,
   contraction_plan,
 ) where {ElR<:Number,NR}
+  if any(b -> !allequal(Tuple(b)), nzblocks(T2))
+    return error(
+      "When contracting a BlockSparse tensor with a DiagBlockSparse tensor, the DiagBlockSparse tensor must be block diagonal for the time being.",
+    )
+  end
   already_written_to = Dict{Block{NR},Bool}()
   indsR = inds(R)
   indsT1 = inds(T1)
diff --git a/NDTensors/test/test_diagblocksparse.jl b/NDTensors/test/test_diagblocksparse.jl
index f47f4b5da0..2aa0e98c24 100644
--- a/NDTensors/test/test_diagblocksparse.jl
+++ b/NDTensors/test/test_diagblocksparse.jl
@@ -1,8 +1,16 @@
 @eval module $(gensym())
 using Dictionaries: Dictionary
-using NDTensors
-using Test: @testset, @test
-
+using NDTensors:
+  NDTensors,
+  Block,
+  BlockSparseTensor,
+  DiagBlockSparse,
+  Tensor,
+  blockoffsets,
+  contract,
+  nzblocks
+using Random: randn!
+using Test: @test, @test_throws, @testset
 @testset "UniformDiagBlockSparseTensor basic functionality" begin
   NeverAlias = NDTensors.NeverAlias
   AllowAlias = NDTensors.AllowAlias
@@ -25,4 +33,19 @@ using Test: @testset, @test
   @test conj(NeverAlias(), tensor)[1, 1] == conj(c)
   @test conj(AllowAlias(), tensor)[1, 1] == conj(c)
 end
+@testset "DiagBlockSparse off-diagonal (eltype=$elt)" for elt in (
+  Float32, Float64, Complex{Float32}, Complex{Float64}
+)
+  inds1 = ([1, 1], [1, 1])
+  inds2 = ([1, 1], [1, 1])
+  blocks = [(1, 2), (2, 1)]
+  a1 = BlockSparseTensor{elt}(blocks, inds1...)
+  for b in nzblocks(a1)
+    randn!(a1[b])
+  end
+  a2 = Tensor(DiagBlockSparse(one(elt), blockoffsets(a1)), inds2)
+  for (labels1, labels2) in (((1, -1), (-1, 2)), ((-1, -2), (-1, -2)))
+    @test_throws ErrorException contract(a1, labels1, a2, labels2)
+  end
+end
 end