diff --git a/src/libFLAC/metadata_iterators.c b/src/libFLAC/metadata_iterators.c
index 9356416..1acb19c 100644
--- a/src/libFLAC/metadata_iterators.c
+++ b/src/libFLAC/metadata_iterators.c
@@ -2536,6 +2536,9 @@ FLAC__bool write_metadata_block_header_cb_(FLAC__IOHandle handle, FLAC__IOCallba
 	FLAC__byte buffer[FLAC__STREAM_METADATA_HEADER_LENGTH];
 
 	FLAC__ASSERT(block->length < (1u << FLAC__STREAM_METADATA_LENGTH_LEN));
+	/* double protection */
+	if(block->length >= (1u << FLAC__STREAM_METADATA_LENGTH_LEN))
+		return false;
 
 	buffer[0] = (block->is_last? 0x80 : 0) | (FLAC__byte)block->type;
 	pack_uint32_(block->length, buffer + 1, 3);
diff --git a/src/libFLAC/stream_encoder_framing.c b/src/libFLAC/stream_encoder_framing.c
index 9faadd2..84e5ae1 100644
--- a/src/libFLAC/stream_encoder_framing.c
+++ b/src/libFLAC/stream_encoder_framing.c
@@ -65,6 +65,9 @@ FLAC__bool FLAC__add_metadata_block(const FLAC__StreamMetadata *metadata, FLAC__
 		i += vendor_string_length;
 	}
 	FLAC__ASSERT(i < (1u << FLAC__STREAM_METADATA_LENGTH_LEN));
+	/* double protection */
+	if(i >= (1u << FLAC__STREAM_METADATA_LENGTH_LEN))
+		return false;
 	if(!FLAC__bitwriter_write_raw_uint32(bw, i, FLAC__STREAM_METADATA_LENGTH_LEN))
 		return false;