diff --git a/libavformat/matroskadec.c b/libavformat/matroskadec.c
index 316394d034..a43cc0bf19 100644
--- a/libavformat/matroskadec.c
+++ b/libavformat/matroskadec.c
@@ -707,7 +707,7 @@ static EbmlSyntax matroska_segments[] = {
 };
 
 static 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) },
     CHILD_OF(matroska_blockadditions)
 };