diff --git a/src/nplinker/genomics/bgc.py b/src/nplinker/genomics/bgc.py
index 77f16543..8decbb81 100644
--- a/src/nplinker/genomics/bgc.py
+++ b/src/nplinker/genomics/bgc.py
@@ -229,17 +229,17 @@ def _to_string(value: Any) -> str:
         """
         # Convert list to comma-separated string
         if isinstance(value, list):
-            value = ", ".join(map(str, value))
+            formatted_value = ", ".join(map(str, value))
         # Convert dict to comma-separated string
         elif isinstance(value, dict):
-            value = ", ".join([f"{k}:{v}" for k, v in value.items()])
+            formatted_value = ", ".join([f"{k}:{v}" for k, v in value.items()])
         # Convert None to empty string
         elif value is None:
-            value = ""
+            formatted_value = ""
         # Convert anything else to string
         else:
-            value = str(value)
-        return value
+            formatted_value = str(value)
+        return formatted_value
 
     # CG: why not providing whole product but only amino acid as product monomer?
     # this property is not used in NPLinker core business.
diff --git a/src/nplinker/metabolomics/spectrum.py b/src/nplinker/metabolomics/spectrum.py
index 2674f461..6928a4f1 100644
--- a/src/nplinker/metabolomics/spectrum.py
+++ b/src/nplinker/metabolomics/spectrum.py
@@ -149,14 +149,14 @@ def _to_string(value: Any) -> str:
         """
         # Convert list to comma-separated string
         if isinstance(value, list):
-            value = ", ".join(map(str, value))
+            formatted_value = ", ".join(map(str, value))
         # Convert dict to comma-separated string
         elif isinstance(value, dict):
-            value = ", ".join([f"{k}:{v}" for k, v in value.items()])
+            formatted_value = ", ".join([f"{k}:{v}" for k, v in value.items()])
         # Convert None to empty string
         elif value is None:
-            value = ""
+            formatted_value = ""
         # Convert anything else to string
         else:
-            value = str(value)
-        return value
+            formatted_value = str(value)
+        return formatted_value