diff --git a/deltachat-ffi/src/lib.rs b/deltachat-ffi/src/lib.rs
index 507fd7de5..0215b8da5 100644
--- a/deltachat-ffi/src/lib.rs
+++ b/deltachat-ffi/src/lib.rs
@@ -30,7 +30,13 @@ pub unsafe extern "C" fn dc_context_new(
 ) -> *mut dc_context_t {
     setup_panic!();
 
+    let os_name = if os_name.is_null() {
+        None
+    } else {
+        Some(dc_tools::to_string_lossy(os_name))
+    };
     let ctx = context::dc_context_new(cb, userdata, os_name);
+
     Box::into_raw(Box::new(ctx))
 }
 
@@ -38,7 +44,8 @@ pub unsafe extern "C" fn dc_context_new(
 pub unsafe extern "C" fn dc_context_unref(context: *mut dc_context_t) {
     assert!(!context.is_null());
     let context = &mut *context;
-    context::dc_context_unref(context)
+    context::dc_context_unref(context);
+    Box::from_raw(context);
 }
 
 #[no_mangle]
diff --git a/examples/repl/main.rs b/examples/repl/main.rs
index 214527467..da7b28cab 100644
--- a/examples/repl/main.rs
+++ b/examples/repl/main.rs
@@ -393,7 +393,7 @@ fn main_0(args: Vec<String>) -> Result<(), failure::Error> {
     let mut context = dc_context_new(
         Some(receive_event),
         0 as *mut libc::c_void,
-        b"CLI\x00" as *const u8 as *const libc::c_char,
+        Some("CLI".into()),
     );
 
     unsafe { dc_cmdline_skip_auth() };
diff --git a/examples/simple.rs b/examples/simple.rs
index 2aa824acb..fc80fda68 100644
--- a/examples/simple.rs
+++ b/examples/simple.rs
@@ -41,7 +41,7 @@ extern "C" fn cb(_ctx: &Context, event: Event, data1: usize, data2: usize) -> us
 
 fn main() {
     unsafe {
-        let ctx = dc_context_new(Some(cb), std::ptr::null_mut(), std::ptr::null_mut());
+        let ctx = dc_context_new(Some(cb), std::ptr::null_mut(), None);
         let running = Arc::new(RwLock::new(true));
         let info = dc_get_info(&ctx);
         let info_s = CStr::from_ptr(info);
diff --git a/src/context.rs b/src/context.rs
index 29d8525a4..0a985447c 100644
--- a/src/context.rs
+++ b/src/context.rs
@@ -35,7 +35,7 @@ pub struct Context {
     pub smtp_state: Arc<(Mutex<SmtpState>, Condvar)>,
     pub oauth2_critical: Arc<Mutex<()>>,
     pub cb: Option<dc_callback_t>,
-    pub os_name: *mut libc::c_char,
+    pub os_name: Option<String>,
     pub cmdline_sel_chat_id: Arc<RwLock<u32>>,
     pub bob: Arc<RwLock<BobStatus>>,
     pub last_smeared_timestamp: Arc<RwLock<i64>>,
@@ -116,7 +116,7 @@ pub struct SmtpState {
 pub fn dc_context_new(
     cb: Option<dc_callback_t>,
     userdata: *mut libc::c_void,
-    os_name: *const libc::c_char,
+    os_name: Option<String>,
 ) -> Context {
     Context {
         blobdir: Arc::new(RwLock::new(std::ptr::null_mut())),
@@ -131,7 +131,7 @@ pub fn dc_context_new(
         })),
         userdata,
         cb,
-        os_name: unsafe { dc_strdup_keep_null(os_name) },
+        os_name: os_name,
         running_state: Arc::new(RwLock::new(Default::default())),
         sql: Sql::new(),
         smtp: Arc::new(Mutex::new(Smtp::new())),
@@ -165,6 +165,16 @@ pub fn dc_context_new(
     }
 }
 
+#[cfg(test)]
+mod tests {
+    use super::*;
+    #[test]
+    fn no_crashes_on_context_deref() {
+        let mut ctx = dc_context_new(None, std::ptr::null_mut(), Some("Test OS".into()));
+        unsafe { dc_context_unref(&mut ctx) };
+    }
+}
+
 unsafe fn cb_receive_imf(
     context: &Context,
     imf_raw_not_terminated: *const libc::c_char,
@@ -252,8 +262,6 @@ pub unsafe fn dc_context_unref(context: &mut Context) {
     if 0 != dc_is_open(context) {
         dc_close(context);
     }
-
-    free(context.os_name as *mut libc::c_void);
 }
 
 pub unsafe fn dc_close(context: &Context) {
diff --git a/src/dc_mimefactory.rs b/src/dc_mimefactory.rs
index e32b31315..a3cccb129 100644
--- a/src/dc_mimefactory.rs
+++ b/src/dc_mimefactory.rs
@@ -477,26 +477,26 @@ pub unsafe fn dc_mimefactory_render(mut factory: *mut dc_mimefactory_t) -> libc:
             references_list,
             0 as *mut libc::c_char,
         );
+
+        let os_name = &(*factory).context.os_name;
+        let os_part = os_name
+            .as_ref()
+            .map(|s| format!("/{}", s))
+            .unwrap_or_default();
+        let os_part = CString::new(os_part).expect("String -> CString conversion failed");
+
         mailimf_fields_add(
             imf_fields,
             mailimf_field_new_custom(
                 strdup(b"X-Mailer\x00" as *const u8 as *const libc::c_char),
                 dc_mprintf(
-                    b"Delta Chat Core %s%s%s\x00" as *const u8 as *const libc::c_char,
+                    b"Delta Chat Core %s%s\x00" as *const u8 as *const libc::c_char,
                     DC_VERSION_STR as *const u8 as *const libc::c_char,
-                    if !(*(*factory).context).os_name.is_null() {
-                        b"/\x00" as *const u8 as *const libc::c_char
-                    } else {
-                        b"\x00" as *const u8 as *const libc::c_char
-                    },
-                    if !(*(*factory).context).os_name.is_null() {
-                        (*(*factory).context).os_name
-                    } else {
-                        b"\x00" as *const u8 as *const libc::c_char
-                    },
+                    os_part.as_ptr(),
                 ),
             ),
         );
+
         mailimf_fields_add(
             imf_fields,
             mailimf_field_new_custom(
diff --git a/src/lib.rs b/src/lib.rs
index 66181db63..bf361ae0d 100644
--- a/src/lib.rs
+++ b/src/lib.rs
@@ -1,4 +1,4 @@
-#![feature(c_variadic, ptr_wrapping_offset_from)]
+#![feature(c_variadic, ptr_wrapping_offset_from, ptr_cast)]
 
 #[macro_use]
 extern crate failure_derive;
diff --git a/src/peerstate.rs b/src/peerstate.rs
index 7d0f2f648..b8f9b7724 100644
--- a/src/peerstate.rs
+++ b/src/peerstate.rs
@@ -517,7 +517,7 @@ mod tests {
     }
 
     unsafe fn create_test_context() -> TestContext {
-        let mut ctx = dc_context_new(Some(cb), std::ptr::null_mut(), std::ptr::null_mut());
+        let mut ctx = dc_context_new(Some(cb), std::ptr::null_mut(), None);
         let dir = tempdir().unwrap();
         let dbfile = dir.path().join("db.sqlite");
         assert!(
diff --git a/src/stock.rs b/src/stock.rs
index 3dcd66dfb..3416c31ea 100644
--- a/src/stock.rs
+++ b/src/stock.rs
@@ -264,7 +264,7 @@ mod tests {
 
     #[test]
     fn test_stock_str() {
-        let ctx = dc_context_new(None, std::ptr::null_mut(), std::ptr::null_mut());
+        let ctx = dc_context_new(None, std::ptr::null_mut(), None);
         assert_eq!(ctx.stock_str(StockMessage::NoMessages), "No messages.");
     }
 
@@ -290,7 +290,7 @@ mod tests {
 
     #[test]
     fn test_stock_string_repl_str() {
-        let ctx = dc_context_new(None, std::ptr::null_mut(), std::ptr::null_mut());
+        let ctx = dc_context_new(None, std::ptr::null_mut(), None);
         // uses %1$s substitution
         assert_eq!(
             ctx.stock_string_repl_str(StockMessage::Member, "42"),
@@ -301,7 +301,7 @@ mod tests {
 
     #[test]
     fn test_stock_string_repl_int() {
-        let ctx = dc_context_new(None, std::ptr::null_mut(), std::ptr::null_mut());
+        let ctx = dc_context_new(None, std::ptr::null_mut(), None);
         assert_eq!(
             ctx.stock_string_repl_int(StockMessage::Member, 42),
             "42 member(s)"
@@ -310,7 +310,7 @@ mod tests {
 
     #[test]
     fn test_stock_string_repl_str2() {
-        let ctx = dc_context_new(None, std::ptr::null_mut(), std::ptr::null_mut());
+        let ctx = dc_context_new(None, std::ptr::null_mut(), None);
         assert_eq!(
             ctx.stock_string_repl_str2(StockMessage::ServerResponse, "foo", "bar"),
             "Response from foo: bar"
@@ -319,7 +319,7 @@ mod tests {
 
     #[test]
     fn test_stock_system_msg_simple() {
-        let ctx = dc_context_new(None, std::ptr::null_mut(), std::ptr::null_mut());
+        let ctx = dc_context_new(None, std::ptr::null_mut(), None);
         assert_eq!(
             ctx.stock_system_msg(StockMessage::MsgLocationEnabled, "", "", 0),
             "Location streaming enabled."
@@ -328,7 +328,7 @@ mod tests {
 
     #[test]
     fn test_stock_system_msg_add_member_by_me() {
-        let ctx = dc_context_new(None, std::ptr::null_mut(), std::ptr::null_mut());
+        let ctx = dc_context_new(None, std::ptr::null_mut(), None);
         assert_eq!(
             ctx.stock_system_msg(
                 StockMessage::MsgAddMember,
diff --git a/src/test_utils.rs b/src/test_utils.rs
index f78aa14e6..5ba5c0a37 100644
--- a/src/test_utils.rs
+++ b/src/test_utils.rs
@@ -24,7 +24,7 @@ pub struct TestContext {
 /// [Context]: crate::context::Context
 pub fn test_context(cb: Option<dc_callback_t>) -> TestContext {
     unsafe {
-        let mut ctx = dc_context_new(cb, std::ptr::null_mut(), std::ptr::null_mut());
+        let mut ctx = dc_context_new(cb, std::ptr::null_mut(), None);
         let dir = tempdir().unwrap();
         let dbfile = dir.path().join("db.sqlite");
         assert!(
diff --git a/tests/stress.rs b/tests/stress.rs
index 5b3408686..4f363cb5e 100644
--- a/tests/stress.rs
+++ b/tests/stress.rs
@@ -802,7 +802,7 @@ struct TestContext {
 }
 
 unsafe fn create_test_context() -> TestContext {
-    let mut ctx = dc_context_new(Some(cb), std::ptr::null_mut(), std::ptr::null_mut());
+    let mut ctx = dc_context_new(Some(cb), std::ptr::null_mut(), None);
     let dir = tempdir().unwrap();
     let dbfile = dir.path().join("db.sqlite");
     assert!(
@@ -986,7 +986,7 @@ fn test_chat() {
 #[test]
 fn test_wrong_db() {
     unsafe {
-        let mut ctx = dc_context_new(Some(cb), std::ptr::null_mut(), std::ptr::null_mut());
+        let mut ctx = dc_context_new(Some(cb), std::ptr::null_mut(), None);
         let dir = tempdir().unwrap();
         let dbfile = dir.path().join("db.sqlite");
         std::fs::write(&dbfile, b"123").unwrap();