diff --git a/src/ytdl_sub/script/types/syntax_tree.py b/src/ytdl_sub/script/types/syntax_tree.py
index 18ac7579..b49bd6cf 100644
--- a/src/ytdl_sub/script/types/syntax_tree.py
+++ b/src/ytdl_sub/script/types/syntax_tree.py
@@ -45,40 +45,30 @@ class SyntaxTree(VariableDependency):
         return String("".join([str(res) for res in resolved]))
 
     @classmethod
-    def _get_custom_function_dependencies(
+    def _traverse_custom_function_dependencies(
         cls,
         custom_function_name: str,
         custom_function_dependency: "SyntaxTree",
         custom_functions: Dict[str, "SyntaxTree"],
         deps: List[str],
-    ) -> List[str]:
-        deps = copy.deepcopy(deps)  # do not work with references
-
+    ) -> None:
         for dep in custom_function_dependency.custom_functions:
-            # Skip leaf functions since they will never cause a cycle
-            if not custom_functions[dep.name].custom_functions:
-                continue
-
-            deps.append(dep.name)
-
-            if custom_function_name in deps:
-                cycle_deps = [custom_function_name] + deps[0 : deps.index(custom_function_name) + 1]
+            if custom_function_name in deps + [dep.name]:
+                cycle_deps = [custom_function_name] + deps + [dep.name]
                 cycle_deps_str = " -> ".join([f"%{name}" for name in cycle_deps])
                 raise CycleDetected(f"Custom functions contain a cycle: {cycle_deps_str}")
 
-            deps += cls._get_custom_function_dependencies(
+            cls._traverse_custom_function_dependencies(
                 custom_function_name=custom_function_name,
                 custom_function_dependency=custom_functions[dep.name],
                 custom_functions=custom_functions,
-                deps=deps,
+                deps=deps + [dep.name],
             )
 
-        return deps
-
     @classmethod
     def _ensure_no_custom_function_cycles(cls, custom_functions: Dict[str, "SyntaxTree"]):
         for custom_function_name, custom_function in custom_functions.items():
-            _ = cls._get_custom_function_dependencies(
+            cls._traverse_custom_function_dependencies(
                 custom_function_name=custom_function_name,
                 custom_function_dependency=custom_function,
                 custom_functions=custom_functions,