diff --git a/medcat/utils/preprocess_snomed.py b/medcat/utils/preprocess_snomed.py
index 5e65b3a77..3ba94b977 100644
--- a/medcat/utils/preprocess_snomed.py
+++ b/medcat/utils/preprocess_snomed.py
@@ -327,7 +327,7 @@ def _check_path_and_release(self):
             raise FileNotFoundError('Incorrect path to SNOMED CT directory')
         return paths, snomed_releases
 
-    def _refset_df2dict(refset_df: pd.DataFrame) -> dict:
+    def _refset_df2dict(self, refset_df: pd.DataFrame) -> dict:
         """
         This function takes a SNOMED refset DataFrame as an input and converts it into a dictionary.
         The DataFrame should contain the columns 'referencedComponentId','mapTarget','mapGroup','mapPriority','mapRule','mapAdvice'.