diff --git a/rdkit-sys/tests/test_atoms.rs b/rdkit-sys/tests/test_atoms.rs
index b46da68..f5d9d5c 100644
--- a/rdkit-sys/tests/test_atoms.rs
+++ b/rdkit-sys/tests/test_atoms.rs
@@ -1,7 +1,7 @@
 #[test]
 fn test_atoms() {
     cxx::let_cxx_string!(smiles = "c1ccccc1CCCCCCCC");
-    let romol = rdkit_sys::ro_mol_ffi::smiles_to_mol(&smiles).unwrap();
+    let mut romol = rdkit_sys::ro_mol_ffi::smiles_to_mol(&smiles).unwrap();
 
     let num_atoms = rdkit_sys::ro_mol_ffi::get_num_atoms(&romol, true);
 
diff --git a/src/graphmol/rw_mol.rs b/src/graphmol/rw_mol.rs
index 6467120..9c3b5f7 100644
--- a/src/graphmol/rw_mol.rs
+++ b/src/graphmol/rw_mol.rs
@@ -9,6 +9,14 @@ pub struct RWMol {
     pub(crate) ptr: SharedPtr<rdkit_sys::rw_mol_ffi::RWMol>,
 }
 
+#[derive(Debug, PartialEq, thiserror::Error)]
+pub enum RWMolError {
+    #[error("Could not convert smarts to RWMol (nullptr)")]
+    UnknownConversionError,
+    #[error("could not convert smarts to RWMol (exception)")]
+    ConversionException(String),
+}
+
 impl RWMol {
     pub fn from_mol_block(
         mol_block: &str,
@@ -48,11 +56,20 @@ impl RWMol {
         ROMol { ptr }
     }
 
-    pub fn from_smarts(smarts: &str) -> Result<Self, Box<dyn std::error::Error>> {
+    pub fn from_smarts(smarts: &str) -> Result<Self, RWMolError> {
         let_cxx_string!(smarts = smarts);
 
-        let ptr = rdkit_sys::rw_mol_ffi::smarts_to_mol(&smarts)?;
-        Ok(RWMol { ptr })
+        let ptr = rdkit_sys::rw_mol_ffi::smarts_to_mol(&smarts);
+        match ptr {
+            Ok(ptr) => {
+                if ptr.is_null() {
+                    Err(RWMolError::UnknownConversionError)
+                } else {
+                    Ok(RWMol { ptr })
+                }
+            }
+            Err(e) => Err(RWMolError::ConversionException(e.to_string())),
+        }
     }
 }
 
diff --git a/tests/test_atom.rs b/tests/test_atom.rs
index 356aebc..8e2deb8 100644
--- a/tests/test_atom.rs
+++ b/tests/test_atom.rs
@@ -1,6 +1,6 @@
 #[test]
 fn test_atom() {
-    let romol = rdkit::ROMol::from_smiles("C").unwrap();
+    let mut romol = rdkit::ROMol::from_smiles("C").unwrap();
 
     let atom = romol.atom_with_idx(0);
 
diff --git a/tests/test_graphmol.rs b/tests/test_graphmol.rs
index df95351..b3d4275 100644
--- a/tests/test_graphmol.rs
+++ b/tests/test_graphmol.rs
@@ -1,7 +1,7 @@
 use rdkit::{
     detect_chemistry_problems, fragment_parent, substruct_match, CleanupParameters,
-    MolSanitizeException, ROMol, ROMolError, RWMol, SmilesParserParams, SubstructMatchParameters,
-    TautomerEnumerator, Uncharger,
+    MolSanitizeException, ROMol, ROMolError, RWMol, RWMolError, SmilesParserParams,
+    SubstructMatchParameters, TautomerEnumerator, Uncharger,
 };
 
 #[test]
@@ -294,3 +294,10 @@ fn test_building_rwmol_from_smarts() {
     let result = substruct_match(&ro_mol, &query_mol, &SubstructMatchParameters::default());
     assert_eq!(result.len(), 0);
 }
+
+#[test]
+fn test_building_rwmol_from_invalid_smarts() {
+    let smarts = "string";
+    let rw_mol = RWMol::from_smarts(smarts);
+    assert_eq!(rw_mol.err(), Some(RWMolError::UnknownConversionError))
+}