diff --git a/src/data.cpp b/src/data.cpp
index f127a12..84dbbd6 100644
--- a/src/data.cpp
+++ b/src/data.cpp
@@ -108,7 +108,7 @@
   size_t totalLength = encoder.size();
   totalLength += encoder.appendBlock(signatureValue);
 
-  totalLength += encoder.prependVarNumber(totalLength);
+  encoder.prependVarNumber(totalLength);
   encoder.prependVarNumber(tlv::Data);
 
   const_cast<Data*>(this)->wireDecode(encoder.block());