diff --git a/src/dc_receive_imf.rs b/src/dc_receive_imf.rs
index b52baa0b7..21010d955 100644
--- a/src/dc_receive_imf.rs
+++ b/src/dc_receive_imf.rs
@@ -948,10 +948,10 @@ unsafe fn create_or_lookup_group(
         if !field.is_null() && (*field).fld_type == MAILIMF_FIELD_MESSAGE_ID as libc::c_int {
             let fld_message_id: *mut mailimf_message_id = (*field).fld_data.fld_message_id;
             if !fld_message_id.is_null() {
-                if let Some(_grpid) =
+                if let Some(extracted_grpid) =
                     dc_extract_grpid_from_rfc724_mid(as_str((*fld_message_id).mid_value))
                 {
-                    grpid = to_cstring(_grpid);
+                    grpid = extracted_grpid.strdup();
                 } else {
                     grpid = 0 as *mut libc::c_char;
                 }
diff --git a/src/dc_tools.rs b/src/dc_tools.rs
index e1e5423e6..f84d36d33 100644
--- a/src/dc_tools.rs
+++ b/src/dc_tools.rs
@@ -832,7 +832,7 @@ pub unsafe fn dc_extract_grpid_from_rfc724_mid_list(list: *const clist) -> *mut
             };
 
             if let Some(grpid) = dc_extract_grpid_from_rfc724_mid(mid) {
-                return to_cstring(grpid);
+                return grpid.strdup();
             }
             cur = if !cur.is_null() {
                 (*cur).next
@@ -2113,24 +2113,24 @@ mod tests {
 
     #[test]
     fn test_dc_extract_grpid_from_rfc724_mid() {
-        // Should return 0 if we pass invalid mid
-        let str = "foobar";
-        let grpid = dc_extract_grpid_from_rfc724_mid(str);
+        // Should return None if we pass invalid mid
+        let mid = "foobar";
+        let grpid = dc_extract_grpid_from_rfc724_mid(mid);
         assert_eq!(grpid, None);
 
-        // Should return 0 if grpid has a length which is not 11 or 16
-        let str = "Gr.12345678.morerandom@domain.de";
-        let grpid = dc_extract_grpid_from_rfc724_mid(str);
+        // Should return None if grpid has a length which is not 11 or 16
+        let mid = "Gr.12345678.morerandom@domain.de";
+        let grpid = dc_extract_grpid_from_rfc724_mid(mid);
         assert_eq!(grpid, None);
 
         // Should return extracted grpid for grpid with length of 11
-        let str = "Gr.12345678901.morerandom@domain.de";
-        let grpid = dc_extract_grpid_from_rfc724_mid(str);
+        let mid = "Gr.12345678901.morerandom@domain.de";
+        let grpid = dc_extract_grpid_from_rfc724_mid(mid);
         assert_eq!(grpid, Some("12345678901"));
 
         // Should return extracted grpid for grpid with length of 11
-        let str = "Gr.1234567890123456.morerandom@domain.de";
-        let grpid = dc_extract_grpid_from_rfc724_mid(str);
+        let mid = "Gr.1234567890123456.morerandom@domain.de";
+        let grpid = dc_extract_grpid_from_rfc724_mid(mid);
         assert_eq!(grpid, Some("1234567890123456"));
     }