diff --git a/drools-drl/drools-drl-parser/src/main/java/org/drools/drl/parser/antlr4/DRLVisitorImpl.java b/drools-drl/drools-drl-parser/src/main/java/org/drools/drl/parser/antlr4/DRLVisitorImpl.java
index 7001beed648..485928d6e7a 100644
--- a/drools-drl/drools-drl-parser/src/main/java/org/drools/drl/parser/antlr4/DRLVisitorImpl.java
+++ b/drools-drl/drools-drl-parser/src/main/java/org/drools/drl/parser/antlr4/DRLVisitorImpl.java
@@ -1087,9 +1087,10 @@ public BaseDescr visitLhsUnary(DRLParser.LhsUnaryContext ctx) {
                     .build();
             children.forEach(andDescr::addDescr);
             return andDescr;
-        } else {
-            // size == 1. children never be empty
+        } else if (children.size() == 1) {
             return children.get(0);
+        } else {
+            return null; // only caused by a parser error
         }
     }