diff --git a/ogc/bblocks/util.py b/ogc/bblocks/util.py
index fa5f0fa..ff60841 100644
--- a/ogc/bblocks/util.py
+++ b/ogc/bblocks/util.py
@@ -788,8 +788,10 @@ def walk(subschema: dict | list, schema_id: str | Path, is_properties: bool = Fa
                 apply_fixes(subschema)
 
             schema_version = subschema.pop('$schema', None)
-            schema_id = subschema.pop('$id', schema_id)
-            if isinstance(schema_id, str) and isinstance(subschema.get('$ref'), str):
+            schema_declared_id = subschema.pop('$id')
+            if schema_declared_id:
+                schema_id = schema_declared_id
+            if isinstance(schema_id, (str, Path)) and isinstance(subschema.get('$ref'), str):
 
                 ref = f"{schema_url}#/x-defs/{get_ref_mapping(schema_id, subschema.pop('$ref'))}"