diff --git a/ersilia/utils/identifiers/compound.py b/ersilia/utils/identifiers/compound.py
index 18c98667b..bbe1c44fb 100644
--- a/ersilia/utils/identifiers/compound.py
+++ b/ersilia/utils/identifiers/compound.py
@@ -56,6 +56,8 @@ def is_key_header(self, h):
         return h.lower() in self.key_header_synonyms
 
     def _is_smiles(self, text):
+        if not isinstance(text, str) or not text.strip():
+            return False  
         if self.Chem is None:
             return asyncio.run(self._process_pubchem_inchikey(text)) is not None
         else:
diff --git a/test/test_compound_identifier.py b/test/test_compound_identifier.py
index 7247a4789..228fa35db 100644
--- a/test/test_compound_identifier.py
+++ b/test/test_compound_identifier.py
@@ -1,3 +1,5 @@
+
+from ersilia.default import UNPROCESSABLE_INPUT
 import pytest
 from ersilia.utils.identifiers.compound import CompoundIdentifier
 from unittest.mock import patch
@@ -38,29 +40,89 @@ def test_is_inchikey_positive(compound_identifier, inchikey):
     """Test that valid InChIKeys return True."""
     assert compound_identifier._is_inchikey(inchikey) is True
 
+@pytest.fixture(params=[True, False], ids=["Chem_None", "Chem_Not_None"])
+def compound_identifier(request):
+    """Fixture that initializes CompoundIdentifier with or without RDKit."""
+    return CompoundIdentifier(local=request.param)
 
-@pytest.mark.parametrize("inchikey", [
-    "BSYNRYMUTXBXSQUHFFFAOYSA",        
-    "BSYNRYMUTXBXSQ-UHFFFAOYSA-XY", 
-    "12345678901234-1234567890-X",
-    "BSYNRYMUTXBXSQ_UHFFFAOYSA-N",
-    "BSYNRYMUTXBXSQ-UHFFFAOYSA"
+@pytest.mark.parametrize("smiles, expected", [
+    ("C", True), 
+    ("CCO", True) 
 ])
-def test_is_inchikey_negative(compound_identifier, inchikey):
-    """Test that invalid InChIKeys return False."""
-    assert not compound_identifier._is_inchikey(inchikey)
+def test_is_smiles_positive(compound_identifier, smiles, expected):
+    """Test _is_smiles returns True for valid SMILES strings."""
+    if compound_identifier.Chem is None:
+        assert compound_identifier._is_smiles(smiles) == expected
 
-    
-def test_guess_type_with_inchikey(compound_identifier):
-    inchikey = "LFQSCWFLJHTTHZ-UHFFFAOYSA-N"
+@pytest.mark.parametrize("smiles, expected", [
+    ("invalid_smiles", False),  
+    ("", False)  
+])
+def test_is_smiles_negative(compound_identifier, smiles, expected):
+    """Test _is_smiles returns False for invalid or empty SMILES strings."""
+    assert compound_identifier._is_smiles(smiles) == expected
+
+@pytest.mark.parametrize("inchikey, expected", [
+    ("BQJCRHHNABKAKU-KBQPJGBKSA-N", True), 
+])
+def test_is_inchikey_positive(inchikey, expected):
+    """Test _is_inchikey returns True for valid InChIKey."""
+    assert CompoundIdentifier._is_inchikey(inchikey) == expected
+
+@pytest.mark.parametrize("inchikey, expected", [
+    ("invalid_inchikey", False),
+    ("BQJCRHHNABKAKU-KBQPJGBKSA", False) 
+])
+def test_is_inchikey_negative(inchikey, expected):
+    """Test _is_inchikey returns False for invalid InChIKeys."""
+    assert CompoundIdentifier._is_inchikey(inchikey) == expected
+
+@pytest.mark.parametrize("inchikey, expected", [
+    ("BQJCRHHNABKAKU-KBQPJGBKSA-N", "inchikey"),  
+    ("ABCDEFGHIJKLMN-OPQRSTUVWX-Y", "inchikey"),  
+])
+def test_guess_type_inchikey(compound_identifier, inchikey, expected):
+    """Ensure guess_type correctly identifies valid InChIKeys."""
     result = compound_identifier.guess_type(inchikey)
-    assert result == "inchikey"
-    
-    
-@patch('ersilia.utils.identifiers.compound.CompoundIdentifier._pubchem_smiles_to_inchikey')
-def test_is_smiles_positive_chem_none(mock_pubchem, compound_identifier):
-    compound_identifier.Chem = None
-    mock_pubchem.return_value = "InChIKey"
+    assert result == expected, f"Expected 'inchikey', but got '{result}' for input '{inchikey}'"
+
+@pytest.mark.parametrize("smiles, expected", [
+    ("C", "smiles"),  
+    ("CCO", "smiles"), 
+])
+def test_guess_type_smiles(compound_identifier, smiles, expected):
+    """Ensure guess_type correctly identifies valid SMILES strings."""
+    result = compound_identifier.guess_type(smiles)
+    assert result == expected, f"Expected 'smiles', but got '{result}' for input '{smiles}'"
+
+@pytest.mark.parametrize("input_data, expected", [
+    (None, UNPROCESSABLE_INPUT),  
+    (UNPROCESSABLE_INPUT, UNPROCESSABLE_INPUT),  
+])
+def test_guess_type_unprocessable(compound_identifier, input_data, expected):
+    """Ensure guess_type returns UNPROCESSABLE_INPUT for None or unprocessable inputs."""
+    result = compound_identifier.guess_type(input_data)
+    assert result == expected, f"Expected '{UNPROCESSABLE_INPUT}', but got '{result}'"
+
+@pytest.mark.parametrize("whitespace_input, expected", [
+    ("\n", UNPROCESSABLE_INPUT),  
+    ("\t", UNPROCESSABLE_INPUT),  
+    (" ", UNPROCESSABLE_INPUT),   
+])
+def test_guess_type_whitespace(compound_identifier, whitespace_input, expected):
+    """Ensure guess_type returns UNPROCESSABLE_INPUT for whitespace-only input."""
+    result = compound_identifier.guess_type(whitespace_input)
+    assert result == expected, f"Expected '{UNPROCESSABLE_INPUT}' for input '{whitespace_input}'"
+
+@pytest.mark.parametrize("non_char_input, expected", [
+    (12345, UNPROCESSABLE_INPUT),  
+    (3.14, UNPROCESSABLE_INPUT),   
+    ("𠜎𠜱𡿺𠬠", UNPROCESSABLE_INPUT),  
+])
+def test_guess_type_non_character(compound_identifier, non_char_input, expected):
+    """Ensure guess_type returns UNPROCESSABLE_INPUT for non-character input."""
+    result = compound_identifier.guess_type(non_char_input)
+    assert result == expected, f"Expected '{UNPROCESSABLE_INPUT}' for input '{non_char_input}'"
     
 # Test with a valid SMILES input
     smiles_string = 'CCO' #Ethanol SMILES