diff --git a/lib/src/sparql/plan_builder.rs b/lib/src/sparql/plan_builder.rs
index d223f213..401c2be2 100644
--- a/lib/src/sparql/plan_builder.rs
+++ b/lib/src/sparql/plan_builder.rs
@@ -1128,9 +1128,8 @@ impl<'a> PlanBuilder<'a> {
             } => {
                 let mut child_problematic_set = BTreeSet::new();
                 for (child_i, output_i) in lateral_mapping.iter() {
-                    if set.contains(output_i) {
-                        child_problematic_set.insert(*child_i);
-                    }
+                    set.insert(*output_i);
+                    child_problematic_set.insert(*child_i);
                 }
                 Self::add_left_join_problematic_variables(child, &mut child_problematic_set);
                 for (child_i, output_i) in mapping.iter() {
diff --git a/testsuite/oxigraph-tests/sparql/lateral/subselect_inside_optional.srx b/testsuite/oxigraph-tests/sparql/lateral/subselect_inside_optional.srx
index 11630eb9..ce409249 100644
--- a/testsuite/oxigraph-tests/sparql/lateral/subselect_inside_optional.srx
+++ b/testsuite/oxigraph-tests/sparql/lateral/subselect_inside_optional.srx
@@ -26,7 +26,7 @@
         <uri>http://example.org/s2</uri>
       </binding>
       <binding name="o">
-        <literal datatype="http://www.w3.org/2001/XMLSchema#integer">21</literal>
+        <literal datatype="http://www.w3.org/2001/XMLSchema#integer">11</literal>
       </binding>
     </result>
     <result>
@@ -34,13 +34,24 @@
         <uri>http://example.org/s2</uri>
       </binding>
       <binding name="o">
-        <literal datatype="http://www.w3.org/2001/XMLSchema#integer">22</literal>
+        <literal datatype="http://www.w3.org/2001/XMLSchema#integer">11</literal>
       </binding>
     </result>
     <result>
       <binding name="s">
         <uri>http://example.org/s3</uri>
       </binding>
+      <binding name="o">
+        <literal datatype="http://www.w3.org/2001/XMLSchema#integer">11</literal>
+      </binding>
+    </result>
+    <result>
+      <binding name="s">
+        <uri>http://example.org/s3</uri>
+      </binding>
+      <binding name="o">
+        <literal datatype="http://www.w3.org/2001/XMLSchema#integer">11</literal>
+      </binding>
     </result>
   </results>
 </sparql>
\ No newline at end of file