diff --git a/scripts/build_powerplants.py b/scripts/build_powerplants.py
index cefc0ee5..1a714b39 100755
--- a/scripts/build_powerplants.py
+++ b/scripts/build_powerplants.py
@@ -104,7 +104,7 @@ def replace_natural_gas_technology(df):
 
 
 def replace_natural_gas_fueltype(df):
-    return df.Fueltype.mask(df.Technology == "OCGT" | df.Technology == "CCGT", "Natural Gas")
+    return df.Fueltype.mask((df.Technology == "OCGT") | (df.Technology == "CCGT"), "Natural Gas")
 
 
 if __name__ == "__main__":