diff --git a/docs/mapping.html b/docs/mapping.html
index 472de24..b69fec0 100644
--- a/docs/mapping.html
+++ b/docs/mapping.html
@@ -88,8 +88,8 @@ <h1 class="title">Module <code>keboola.json_to_csv.mapping</code></h1>
 
             for child_key, child_mapping in mapping.child_tables.items():
                 # TODO: use dynamic separator
-                key = f&#39;{key}_{child_key}&#39;
-                flat_mappings.update(_flatten_mapping(child_mapping, key))
+                new_key = f&#39;{key}_{child_key}&#39;
+                flat_mappings.update(_flatten_mapping(child_mapping, new_key))
 
             return flat_mappings
 
@@ -283,8 +283,8 @@ <h2 id="attributes">Attributes</h2>
 
             for child_key, child_mapping in mapping.child_tables.items():
                 # TODO: use dynamic separator
-                key = f&#39;{key}_{child_key}&#39;
-                flat_mappings.update(_flatten_mapping(child_mapping, key))
+                new_key = f&#39;{key}_{child_key}&#39;
+                flat_mappings.update(_flatten_mapping(child_mapping, new_key))
 
             return flat_mappings
 
@@ -567,8 +567,8 @@ <h3>Methods</h3>
 
         for child_key, child_mapping in mapping.child_tables.items():
             # TODO: use dynamic separator
-            key = f&#39;{key}_{child_key}&#39;
-            flat_mappings.update(_flatten_mapping(child_mapping, key))
+            new_key = f&#39;{key}_{child_key}&#39;
+            flat_mappings.update(_flatten_mapping(child_mapping, new_key))
 
         return flat_mappings
 
diff --git a/src/keboola/json_to_csv/mapping.py b/src/keboola/json_to_csv/mapping.py
index e6b66b0..a488ca3 100644
--- a/src/keboola/json_to_csv/mapping.py
+++ b/src/keboola/json_to_csv/mapping.py
@@ -60,8 +60,8 @@ def _flatten_mapping(mapping: 'TableMapping', key='') -> Dict:
 
             for child_key, child_mapping in mapping.child_tables.items():
                 # TODO: use dynamic separator
-                key = f'{key}_{child_key}'
-                flat_mappings.update(_flatten_mapping(child_mapping, key))
+                new_key = f'{key}_{child_key}'
+                flat_mappings.update(_flatten_mapping(child_mapping, new_key))
 
             return flat_mappings