diff --git a/format/mp4/boxes.go b/format/mp4/boxes.go
index 55e30b13..c8ea761a 100644
--- a/format/mp4/boxes.go
+++ b/format/mp4/boxes.go
@@ -1723,9 +1723,14 @@ func decodeBox(ctx *decodeContext, d *decode.D, typ string) {
 			// TODO: better probe? ffmpeg uses box name heuristics?
 			// if 16 length field seems to match assume box with length, language and value
 			// otherwise decode as box with value rest of box
-			probeLength := d.PeekUintBits(16)
+
+			// only probe if there is something
+			probeLength := int64(0)
+			if d.BitsLeft() >= 16 {
+				probeLength = int64(d.PeekUintBits(16))
+			}
 			// +2 for length field, +2 for language field
-			if (probeLength+2+2)*8 == uint64(d.BitsLeft()) {
+			if (probeLength+2+2)*8 == d.BitsLeft() {
 				length := d.FieldU16("length")
 				d.FieldStrFn("language", decodeLang)
 				d.FieldUTF8("value", int(length))
diff --git a/format/mp4/testdata/udta_xtra_empty b/format/mp4/testdata/udta_xtra_empty
new file mode 100644
index 00000000..825839df
Binary files /dev/null and b/format/mp4/testdata/udta_xtra_empty differ
diff --git a/format/mp4/testdata/udta_xtra_empty.fqtest b/format/mp4/testdata/udta_xtra_empty.fqtest
new file mode 100644
index 00000000..a41af856
--- /dev/null
+++ b/format/mp4/testdata/udta_xtra_empty.fqtest
@@ -0,0 +1,32 @@
+$ fq -d mp4 -o force=true dv udta_xtra_empty
+    |00 01 02 03 04 05 06 07 08 09 0a 0b 0c 0d 0e 0f|0123456789abcdef|.{}: udta_xtra_empty (mp4) 0x0-0x44.7 (69)
+    |                                               |                |  boxes[0:1]: 0x0-0x44.7 (69)
+    |                                               |                |    [0]{}: box 0x0-0x44.7 (69)
+0x00|00 00 00 45                                    |...E            |      size: 69 0x0-0x3.7 (4)
+0x00|            75 64 74 61                        |    udta        |      type: "udta" (User-data) 0x4-0x7.7 (4)
+    |                                               |                |      boxes[0:2]: 0x8-0x44.7 (61)
+    |                                               |                |        [0]{}: box 0x8-0x3c.7 (53)
+0x00|                        00 00 00 35            |        ...5    |          size: 53 0x8-0xb.7 (4)
+0x00|                                    6d 65 74 61|            meta|          type: "meta" (Metadata container) 0xc-0xf.7 (4)
+0x10|00 00 00 00                                    |....            |          maybe_flags: 0 0x10-0x13.7 (4)
+    |                                               |                |          boxes[0:2]: 0x14-0x3c.7 (41)
+    |                                               |                |            [0]{}: box 0x14-0x34.7 (33)
+0x10|            00 00 00 21                        |    ...!        |              size: 33 0x14-0x17.7 (4)
+0x10|                        68 64 6c 72            |        hdlr    |              type: "hdlr" (Handler, declares the media (handler) type) 0x18-0x1b.7 (4)
+0x10|                                    00         |            .   |              version: 0 0x1c-0x1c.7 (1)
+0x10|                                       00 00 00|             ...|              flags: 0 0x1d-0x1f.7 (3)
+0x20|00 00 00 00                                    |....            |              component_type: "" 0x20-0x23.7 (4)
+0x20|            6d 64 69 72                        |    mdir        |              component_subtype: "mdir" (Metadata) 0x24-0x27.7 (4)
+0x20|                        00 00 00 00            |        ....    |              component_manufacturer: "" 0x28-0x2b.7 (4)
+0x20|                                    00 00 00 00|            ....|              component_flags: 0 0x2c-0x2f.7 (4)
+0x30|00 00 00 00                                    |....            |              component_flags_mask: 0 0x30-0x33.7 (4)
+0x30|            00                                 |    .           |              component_name: "" 0x34-0x34.7 (1)
+    |                                               |                |            [1]{}: box 0x35-0x3c.7 (8)
+0x30|               00 00 00 08                     |     ....       |              size: 8 0x35-0x38.7 (4)
+0x30|                           69 6c 73 74         |         ilst   |              type: "ilst" 0x39-0x3c.7 (4)
+    |                                               |                |              boxes[0:0]: 0x3d-NA (0)
+    |                                               |                |        [1]{}: box 0x3d-0x44.7 (8)
+0x30|                                       00 00 00|             ...|          size: 8 0x3d-0x40.7 (4)
+0x40|08                                             |.               |
+0x40|   58 74 72 61|                                | Xtra|          |          type: "Xtra" 0x41-0x44.7 (4)
+    |                                               |                |          value: "" 0x45-NA (0)