diff --git a/examples/vumps/vumps_2d_heisenberg.jl b/examples/vumps/vumps_2d_heisenberg.jl
index 35118d1..b66b94e 100644
--- a/examples/vumps/vumps_2d_heisenberg.jl
+++ b/examples/vumps/vumps_2d_heisenberg.jl
@@ -34,9 +34,9 @@ function ITensorInfiniteMPS.unit_cell_terms(::Model"heisenberg2D"; width)
   opsum = OpSum()
   for i in 1:width
     # Vertical
-    opsum += -0.5, "S+", i, "S-", mod(i, width) + 1
-    opsum += -0.5, "S-", i, "S+", mod(i, width) + 1
-    opsum += "Sz", i, "Sz", mod(i, width) + 1
+    opsum += -0.5, "S+", i, "S-", i + 1
+    opsum += -0.5, "S-", i, "S+", i + 1
+    opsum += "Sz", i, "Sz", i + 1
     # Horizontal
     opsum += -0.5, "S+", i, "S-", i + width
     opsum += -0.5, "S-", i, "S+", i + width
diff --git a/src/models/models.jl b/src/models/models.jl
index ac0b3a2..e159175 100644
--- a/src/models/models.jl
+++ b/src/models/models.jl
@@ -248,7 +248,9 @@ function infinite_terms(opsum::OpSum; kwargs...)
   # check that we don't have terms we will ignore
   dropped = filter(x -> x <= 0, keys(opsum_cell_dict))
   if length(dropped) > 0
-    @warn "The input unit cell terms include terms that are being ignored on sites: $([d for d in dropped])"
+    error(
+      "The input unit cell terms include terms that are being ignored on sites: $([d for d in dropped])",
+    )
   end
 
   # Assumes each site in the unit cell has a term