diff --git a/Cargo.lock b/Cargo.lock
index ce4cc87..87fb515 100644
--- a/Cargo.lock
+++ b/Cargo.lock
@@ -281,7 +281,7 @@ dependencies = [
 
 [[package]]
 name = "rsmime"
-version = "0.3.0"
+version = "0.3.1"
 dependencies = [
  "openssl",
  "pyo3",
diff --git a/Cargo.toml b/Cargo.toml
index 17cae98..2008ef5 100644
--- a/Cargo.toml
+++ b/Cargo.toml
@@ -1,6 +1,6 @@
 [package]
 name = "rsmime"
-version = "0.3.0"
+version = "0.3.1"
 edition = "2021"
 
 [lib]
diff --git a/pyproject.toml b/pyproject.toml
index 8785fbf..85dc79f 100644
--- a/pyproject.toml
+++ b/pyproject.toml
@@ -4,7 +4,7 @@ build-backend = "maturin"
 
 [project]
 name = "rsmime"
-version = "0.3.0"
+version = "0.3.1"
 classifiers = [
     "License :: OSI Approved :: MIT License",
     "Development Status :: 3 - Alpha",
diff --git a/rsmime.pyi b/rsmime.pyi
index f50dc70..a5c742a 100644
--- a/rsmime.pyi
+++ b/rsmime.pyi
@@ -1,2 +1,11 @@
+class ReadCertificateError(Exception):
+    ...
+
+class LoadCertificateError(Exception):
+    ...
+    
+class SignError(Exception):
+    ...
+
 def sign(cert_file: str, key_file: str, data_to_sign: bytes) -> str:
     ...
\ No newline at end of file
diff --git a/src/lib.rs b/src/lib.rs
index 088321d..cf7e4ff 100644
--- a/src/lib.rs
+++ b/src/lib.rs
@@ -37,13 +37,8 @@ pub fn _sign(cert_file: &str, key_file: &str, data_to_sign: &[u8]) -> PyResult<V
 #[pyfunction]
 fn sign(cert_file: &str, key_file: &str, data_to_sign: Vec<u8>) -> PyResult<String> {
     match _sign(cert_file, key_file, &data_to_sign) {
-        Ok(signed_data) => {
-            match String::from_utf8(signed_data) {
-                Ok(signed_string) => Ok(signed_string),
-                Err(err) => Err(PyException::new_err(err))
-            }
-        },
-        Err(err) => Err(PyException::new_err(err.to_string())),
+        Ok(signed_data) => Ok(String::from_utf8(signed_data).expect("Failed to convert to string")),
+        Err(err) => Err(err),
     }
 }