diff --git a/lib/protocol.js b/lib/protocol.js
index 4296cab..ee49cdb 100644
--- a/lib/protocol.js
+++ b/lib/protocol.js
@@ -1218,7 +1218,7 @@ function read(buffer, cursor, fieldInfo, rootNodes) {
 
 function write(value, buffer, offset, fieldInfo, rootNode) {
   if (fieldInfo.condition && !fieldInfo.condition(rootNode)) {
-    return null;
+    return offset;
   }
   var type = types[fieldInfo.type];
   if (!type) {