diff --git a/lib/ModelingToolkitTearing/src/reassemble.jl b/lib/ModelingToolkitTearing/src/reassemble.jl
index ebdb4cd..8db6ac9 100644
--- a/lib/ModelingToolkitTearing/src/reassemble.jl
+++ b/lib/ModelingToolkitTearing/src/reassemble.jl
@@ -425,6 +425,10 @@ function generate_system_equations!(state::TearingState, neweqs::Vector{Equation
               (var_to_diff[i] !== nothing && !isempty(𝑑neighbors(graph, var_to_diff[i]))))
     end
 
+    # NOTE: `state.structure.graph` should be accurate despite `generate_system_equations!`
+    # mutating the graph (when generating differential equations/populating `total_sub`).
+    # If the graph disagrees with symbolic incidence at any point during the execution of
+    # this function, then there is some invalid operation happening on the graph.
     digraph = DiCMOBiGraph{false}(graph, var_eq_matching)
     for (i, scc) in enumerate(var_sccs)
         # note that the `vscc <-> escc` relation is a set-to-set mapping, and not
@@ -967,7 +971,10 @@ function codegen_equation!(eg::EquationGenerator,
         # We will add `neweq.lhs` to `total_sub`, so any equation involving it won't be
         # incident on it. Remove the edges incident on `iv` from the graph, and add
         # the replacement vertices from `ieq` so that the incidence is still correct.
-        for e in 𝑑neighbors(graph, iv)
+        #
+        # The `copy` is necessary because `rem_edge!` will mutate the buffer that we
+        # iterate over.
+        for e in copy(𝑑neighbors(graph, iv))
             e == ieq && continue
             for v in 𝑠neighbors(graph, ieq)
                 add_edge!(graph, e, v)