diff --git a/player_decode.v b/player_decode.v
index c032db7..6bfe7f9 100644
--- a/player_decode.v
+++ b/player_decode.v
@@ -643,9 +643,10 @@ fn (mut vp VideoPlayer) write_video_frame(mut frame VideoPlayerDecodeStreamFrame
 			return error('aligned access unit ${vp.current_frame} exceeds its ${frame.gpu_bitstream_capacity}-byte bitstream-buffer capacity')
 		}
 		if aligned_size > frame.gpu_bitstream_size {
+			padding_size := bitstream_padding_length(aligned_size - frame.gpu_bitstream_size)!
 			unsafe {
 				vmemset(frame.gpu_bitstream_slice_mapped_memory_address + frame.gpu_bitstream_size,
-					0, isize(aligned_size - frame.gpu_bitstream_size))
+					0, padding_size)
 			}
 		}
 		frame.gpu_bitstream_size = aligned_size
@@ -654,6 +655,14 @@ fn (mut vp VideoPlayer) write_video_frame(mut frame VideoPlayerDecodeStreamFrame
 
 type File = os.File
 
+// Current V uses an int byte count for vmemset; reject narrowing overflow.
+fn bitstream_padding_length(size u64) !int {
+	if size > u64(max_int) {
+		return error('bitstream padding exceeds the supported ${max_int}-byte memory-fill length')
+	}
+	return int(size)
+}
+
 // Return next byte in file, but don't move the cursor position
 fn (mut f File) peek() !u8 {
 	pos_bk := f.tell()!
diff --git a/video_player_test.v b/video_player_test.v
index 75d9f14..b1375d1 100644
--- a/video_player_test.v
+++ b/video_player_test.v
@@ -7,6 +7,27 @@ import encoding.hex
 import os
 import antono2.vulkan as vk
 
+fn test_bitstream_padding_length_is_compatible_with_vmemset() {
+	assert bitstream_padding_length(0)! == 0
+	assert bitstream_padding_length(12)! == 12
+	assert bitstream_padding_length(u64(max_int))! == max_int
+	mut bytes := [u8(1), 2, 3, 0xff, 0xff, 6]
+	unsafe {
+		vmemset(byteptr(bytes.data) + 3, 0, bitstream_padding_length(2)!)
+	}
+	assert bytes == [u8(1), 2, 3, 0, 0, 6]
+}
+
+fn test_bitstream_padding_length_rejects_narrowing_overflow() {
+	for size in [u64(max_int) + 1, ~u64(0)] {
+		if _ := bitstream_padding_length(size) {
+			assert false, 'oversized memory-fill length was accepted'
+		} else {
+			assert err.msg().contains('bitstream padding exceeds')
+		}
+	}
+}
+
 fn test_dpb_acquire_uses_a_free_slot() {
 	mut dpb := DPB{
 		reference_usage: [u8(0), 2]