diff --git a/etl/filter_mastermap.py b/etl/filter_mastermap.py
index 66108b78..d540938f 100644
--- a/etl/filter_mastermap.py
+++ b/etl/filter_mastermap.py
@@ -19,7 +19,8 @@ def main(mastermap_path):
 
 
 def filter_mastermap(mm_path):
-    output_path = "{}.filtered.csv".format(str(mm_path).replace(".gml.csv", ""))
+    output_path = str(mm_path).replace(".gml.csv", "")
+    output_path = "{}.filtered.csv".format(output_path)
     output_fieldnames = ('WKT', 'fid', 'descriptiveGroup')
     # Open the input csv with all polygons, buildings and others
     with open(mm_path, 'r') as fh: