diff --git a/src/pythonjsonlogger/core.py b/src/pythonjsonlogger/core.py
index d78873a..70391e7 100644
--- a/src/pythonjsonlogger/core.py
+++ b/src/pythonjsonlogger/core.py
@@ -243,7 +243,7 @@ def format(self, record: logging.LogRecord) -> str:
         # TODO: logging.LogRecord.msg and logging.LogRecord.message in typeshed
         #        are always type of str. We shouldn't need to override that.
         if isinstance(record.msg, dict):
-            message_dict = record.msg
+            message_dict = record.msg.copy()
             record.message = ""
         else:
             record.message = record.getMessage()