diff --git a/vpr/src/base/clustered_netlist_utils.cpp b/vpr/src/base/clustered_netlist_utils.cpp
index a7488d9ba89..8fbdff8b658 100644
--- a/vpr/src/base/clustered_netlist_utils.cpp
+++ b/vpr/src/base/clustered_netlist_utils.cpp
@@ -48,7 +48,12 @@ void ClusterAtomsLookup::init_lookup() {
     for (auto atom_blk_id : atom_ctx.nlist.blocks()) {
         ClusterBlockId clb_index = atom_ctx.lookup.atom_clb(atom_blk_id);
 
-        cluster_atoms[clb_index].push_back(atom_blk_id);
+        /* if this data structure is being built alongside the clustered netlist    */
+        /* e.g. when ingesting and legalizing a flat placement solution, some atoms */
+        /* may not yet be mapped to a valid clb_index                               */
+        if (clb_index != ClusterBlockId::INVALID()) {
+            cluster_atoms[clb_index].push_back(atom_blk_id);
+        }
     }
 }
 
diff --git a/vpr/src/pack/cluster_util.cpp b/vpr/src/pack/cluster_util.cpp
index 0e12305dc70..53e6acc7fc0 100644
--- a/vpr/src/pack/cluster_util.cpp
+++ b/vpr/src/pack/cluster_util.cpp
@@ -3686,6 +3686,11 @@ void init_clb_atoms_lookup(vtr::vector<ClusterBlockId, std::unordered_set<AtomBl
     for (auto atom_blk_id : atom_ctx.nlist.blocks()) {
         ClusterBlockId clb_index = atom_ctx.lookup.atom_clb(atom_blk_id);
 
-        atoms_lookup[clb_index].insert(atom_blk_id);
+        /* if this data structure is being built alongside the clustered netlist    */
+        /* e.g. when ingesting and legalizing a flat placement solution, some atoms */
+        /* may not yet be mapped to a valid clb_index                               */
+        if (clb_index != ClusterBlockId::INVALID()) {
+            atoms_lookup[clb_index].insert(atom_blk_id);
+        }
     }
-}
\ No newline at end of file
+}