diff --git a/nac3core/src/toplevel/mod.rs b/nac3core/src/toplevel/mod.rs
index 72776b4d..bb4ea3db 100644
--- a/nac3core/src/toplevel/mod.rs
+++ b/nac3core/src/toplevel/mod.rs
@@ -349,6 +349,8 @@ impl TopLevelComposer {
     }
 
     /// step 1, analyze the type vars associated with top level class
+    /// note that we make a duplicate of the type var returned by symbol resolver
+    /// since one top level type var may be used at multiple places
     fn analyze_top_level_class_type_var(&mut self) -> Result<(), String> {
         let def_list = &self.definition_ast_list;
         let temp_def_list = self.extract_def_list();
@@ -441,7 +443,7 @@ impl TopLevelComposer {
                             .map(|x| {
                                 // must be type var here after previous check
                                 let dup = duplicate_type_var(unifier, x);
-                                (dup.1, dup.0)
+                                (dup.2, dup.0)
                             })
                             .collect_vec();
 
@@ -458,6 +460,8 @@ impl TopLevelComposer {
     }
 
     /// step 2, base classes.
+    /// now that the type vars of all classes are done, handle base classes and
+    /// put Self class into the ancestors list. We only allow single inheritance
     fn analyze_top_level_class_bases(&mut self) -> Result<(), String> {
         let temp_def_list = self.extract_def_list();
         for (class_def, class_ast) in self.definition_ast_list.iter_mut() {
@@ -479,6 +483,7 @@ impl TopLevelComposer {
             let class_resolver = class_resolver.as_ref().unwrap();
             let class_resolver = class_resolver.deref();
 
+            // only allow single inheritance
             let mut has_base = false;
             for b in class_bases {
                 // type vars have already been handled, so skip on `Generic[...]`
@@ -508,8 +513,12 @@ impl TopLevelComposer {
                     b,
                 )?;
 
-                if let TypeAnnotation::CustomClassKind { .. } = &base_ty {
-                    // TODO: check to prevent cyclic base class
+                if let TypeAnnotation::CustomClassKind { id, .. } = &base_ty {
+                    // check to prevent cyclic base class
+                    let all_base = Self::get_all_base(*id, &temp_def_list);
+                    if all_base.contains(&class_id) {
+                        return Err("cyclic base detected".into());
+                    }
                     class_ancestors.push(base_ty);
                 } else {
                     return Err(
@@ -905,4 +914,32 @@ impl TopLevelComposer {
         }
         Err(format!("no method {} in the current class", method_name))
     }
+
+    /// get all base class def id of a class, including it self
+    fn get_all_base(
+        child: DefinitionId,
+        temp_def_list: &[Arc<RwLock<TopLevelDef>>]
+    ) -> Vec<DefinitionId> {
+        let mut result: Vec<DefinitionId> = Vec::new();
+        let child_def = temp_def_list.get(child.0).unwrap();
+        let child_def = child_def.read();
+        let child_def = child_def.deref();
+
+        if let TopLevelDef::Class { ancestors, .. } = child_def {
+            for a in ancestors {
+                if let TypeAnnotation::CustomClassKind { id, .. } = a {
+                    if *id != child {
+                        result.extend(Self::get_all_base(*id, temp_def_list));
+                    }
+                } else {
+                    unreachable!("must be class type annotation type")
+                }
+            }
+        } else {
+            unreachable!("this function should only be called with class def id as parameter")
+        }
+
+        result.push(child);
+        result
+    }
 }