diff --git a/planetiler-core/src/main/java/com/onthegomap/planetiler/expression/MultiExpression.java b/planetiler-core/src/main/java/com/onthegomap/planetiler/expression/MultiExpression.java
index 484c5693..d1e86d94 100644
--- a/planetiler-core/src/main/java/com/onthegomap/planetiler/expression/MultiExpression.java
+++ b/planetiler-core/src/main/java/com/onthegomap/planetiler/expression/MultiExpression.java
@@ -148,6 +148,7 @@ public record MultiExpression<T> (List<Entry<T>> expressions) implements Simplif
     return new MultiExpression<>(
       expressions.stream()
         .map(entry -> entry(fn.apply(entry.result), entry.expression))
+        .filter(entry -> entry.result != null)
         .toList()
     );
   }