diff --git a/singer_sdk/mapper.py b/singer_sdk/mapper.py
index 177d11af4..0c01654f8 100644
--- a/singer_sdk/mapper.py
+++ b/singer_sdk/mapper.py
@@ -666,6 +666,9 @@ def register_raw_stream_schema(
             ]
 
         for stream_map_key, stream_def in self.stream_maps_dict.items():
+            stream_def = (
+                stream_def.copy() if isinstance(stream_def, dict) else stream_def
+            )
             stream_alias: str = stream_map_key
             source_stream: str = stream_map_key
             if isinstance(stream_def, str) and stream_def != NULL_STRING: