diff --git a/src/main/java/com/google/api/generator/engine/ast/BlockStatement.java b/src/main/java/com/google/api/generator/engine/ast/BlockStatement.java
index 7ee8efa794..b5ff9373fd 100644
--- a/src/main/java/com/google/api/generator/engine/ast/BlockStatement.java
+++ b/src/main/java/com/google/api/generator/engine/ast/BlockStatement.java
@@ -40,6 +40,12 @@ public abstract static class Builder {
 
     public abstract Builder setIsStatic(boolean isStatic);
 
-    public abstract BlockStatement build();
+    abstract BlockStatement autoBuild();
+
+    public BlockStatement build() {
+      BlockStatement blockStatement = autoBuild();
+      NodeValidator.checkNoNullElements(blockStatement.body(), "body", "block statement");
+      return autoBuild();
+    }
   }
 }