diff --git a/slither/tools/flattening/flattening.py b/slither/tools/flattening/flattening.py
index 67b3c00a3..7603f5e93 100644
--- a/slither/tools/flattening/flattening.py
+++ b/slither/tools/flattening/flattening.py
@@ -11,6 +11,7 @@ from slither.core.declarations import SolidityFunction, EnumContract, StructureC
 from slither.core.declarations.contract import Contract
 from slither.core.declarations.function_top_level import FunctionTopLevel
 from slither.core.declarations.top_level import TopLevel
+from slither.core.declarations.solidity_variables import SolidityCustomRevert
 from slither.core.solidity_types import MappingType, ArrayType
 from slither.core.solidity_types.type import Type
 from slither.core.solidity_types.user_defined_type import UserDefinedType
@@ -24,6 +25,7 @@ from slither.tools.flattening.export.export import (
 )
 
 logger = logging.getLogger("Slither-flattening")
+logger.setLevel(logging.INFO)
 
 # index: where to start
 # patch_type:
@@ -75,6 +77,7 @@ class Flattening:
 
         self._get_source_code_top_level(compilation_unit.structures_top_level)
         self._get_source_code_top_level(compilation_unit.enums_top_level)
+        self._get_source_code_top_level(compilation_unit.custom_errors)
         self._get_source_code_top_level(compilation_unit.variables_top_level)
         self._get_source_code_top_level(compilation_unit.functions_top_level)
 
@@ -249,12 +252,14 @@ class Flattening:
         t: Type,
         contract: Contract,
         exported: Set[str],
-        list_contract: List[Contract],
-        list_top_level: List[TopLevel],
+        list_contract: Set[Contract],
+        list_top_level: Set[TopLevel],
     ):
         if isinstance(t, UserDefinedType):
             t_type = t.type
-            if isinstance(t_type, (EnumContract, StructureContract)):
+            if isinstance(t_type, TopLevel):
+                list_top_level.add(t_type)
+            elif isinstance(t_type, (EnumContract, StructureContract)):
                 if t_type.contract != contract and t_type.contract not in exported:
                     self._export_list_used_contracts(
                         t_type.contract, exported, list_contract, list_top_level
@@ -275,8 +280,8 @@ class Flattening:
         self,
         contract: Contract,
         exported: Set[str],
-        list_contract: List[Contract],
-        list_top_level: List[TopLevel],
+        list_contract: Set[Contract],
+        list_top_level: Set[TopLevel],
     ):
         # TODO: investigate why this happen
         if not isinstance(contract, Contract):
@@ -332,19 +337,21 @@ class Flattening:
 
                 for read in ir.read:
                     if isinstance(read, TopLevel):
-                        if read not in list_top_level:
-                            list_top_level.append(read)
-                if isinstance(ir, InternalCall):
-                    function_called = ir.function
-                    if isinstance(function_called, FunctionTopLevel):
-                        list_top_level.append(function_called)
-
-        if contract not in list_contract:
-            list_contract.append(contract)
+                        list_top_level.add(read)
+                if isinstance(ir, InternalCall) and isinstance(ir.function, FunctionTopLevel):
+                    list_top_level.add(ir.function)
+                if (
+                    isinstance(ir, SolidityCall)
+                    and isinstance(ir.function, SolidityCustomRevert)
+                    and isinstance(ir.function.custom_error, TopLevel)
+                ):
+                    list_top_level.add(ir.function.custom_error)
+
+        list_contract.add(contract)
 
     def _export_contract_with_inheritance(self, contract) -> Export:
-        list_contracts: List[Contract] = []  # will contain contract itself
-        list_top_level: List[TopLevel] = []
+        list_contracts: Set[Contract] = set()  # will contain contract itself
+        list_top_level: Set[TopLevel] = set()
         self._export_list_used_contracts(contract, set(), list_contracts, list_top_level)
         path = Path(self._export_path, f"{contract.name}_{uuid.uuid4()}.sol")
 
@@ -401,8 +408,8 @@ class Flattening:
     def _export_with_import(self) -> List[Export]:
         exports: List[Export] = []
         for contract in self._compilation_unit.contracts:
-            list_contracts: List[Contract] = []  # will contain contract itself
-            list_top_level: List[TopLevel] = []
+            list_contracts: Set[Contract] = set()  # will contain contract itself
+            list_top_level: Set[TopLevel] = set()
             self._export_list_used_contracts(contract, set(), list_contracts, list_top_level)
 
             if list_top_level: