diff --git a/rxnmapper/core.py b/rxnmapper/core.py
index 202fbac..260066d 100644
--- a/rxnmapper/core.py
+++ b/rxnmapper/core.py
@@ -128,6 +128,14 @@ def convert_batch_to_attns(
             return_tensors="pt",
         )
         parsed_input = {k: v.to(self.device) for k, v in encoded_ids.items()}
+
+        max_input_length = parsed_input["input_ids"].shape[1]
+        max_supported_by_model = self.model.config.max_position_embeddings
+        if max_input_length > max_supported_by_model:
+            raise ValueError(
+                f"Reaction SMILES has {max_input_length} tokens, should be at most {max_supported_by_model}."
+            )
+
         with torch.no_grad():
             output = self.model(**parsed_input)
         attentions = output[2]
diff --git a/tests/test_mapper.py b/tests/test_mapper.py
index 86789af..e93d3eb 100644
--- a/tests/test_mapper.py
+++ b/tests/test_mapper.py
@@ -137,3 +137,15 @@ def test_reaction_with_asterisks(rxn_mapper: RXNMapper):
 
     results = rxn_mapper.get_attention_guided_atom_maps(rxns, canonicalize_rxns=False)
     assert_correct_maps(results, expected)
+
+
+def test_too_long_reaction_smiles_produce_exception_with_understandable_error_message(
+    rxn_mapper: RXNMapper,
+):
+    # dummy reaction with 1 + 3 + 500 * 2 + 3 + 1 = 1008 tokens
+    rxn = "C=C" + "[C+][C-]" * 500 + ">>CC"
+
+    with pytest.raises(ValueError) as excinfo:
+        _ = rxn_mapper.get_attention_guided_atom_maps([rxn], canonicalize_rxns=False)
+
+    assert "Reaction SMILES has 1008 tokens, should be" in str(excinfo.value)