diff --git a/libavformat/matroskadec.c b/libavformat/matroskadec.c
index 37c9a1c11e..f1e76b8b45 100644
--- a/libavformat/matroskadec.c
+++ b/libavformat/matroskadec.c
@@ -684,7 +684,7 @@ static const EbmlSyntax matroska_segments[] = {
 };
 
 static const EbmlSyntax matroska_blockmore[] = {
-    { MATROSKA_ID_BLOCKADDID,      EBML_UINT, 0, offsetof(MatroskaBlock,additional_id) },
+    { MATROSKA_ID_BLOCKADDID,      EBML_UINT, 0, offsetof(MatroskaBlock,additional_id), { .u = 1 } },
     { MATROSKA_ID_BLOCKADDITIONAL, EBML_BIN,  0, offsetof(MatroskaBlock,additional) },
     { 0 }
 };