diff --git a/atlasprint/core.py b/atlasprint/core.py
index c8fb9de..91678d3 100755
--- a/atlasprint/core.py
+++ b/atlasprint/core.py
@@ -319,7 +319,7 @@ def optimize_expression(layer, expression):
         return expression
 
     # extract primary key from fields list
-    pk_index = primary_keys[0];
+    pk_index = primary_keys[0]
     pk_field = layer.fields().at(pk_index)
 
     # replace `$id` with effective PK name