diff --git a/mp4/co64.go b/mp4/co64.go
index 1d406539..7034766c 100644
--- a/mp4/co64.go
+++ b/mp4/co64.go
@@ -59,7 +59,7 @@ func (b *Co64Box) Type() string {
 }
 
 func (b *Co64Box) expectedSize(nrEntries uint32) uint64 {
-	return uint64(boxHeaderSize + 8 + nrEntries*8)
+	return uint64(boxHeaderSize + 8 + uint64(nrEntries)*8)
 }
 
 // Size - box-specific size
diff --git a/mp4/testdata/fuzz/FuzzDecodeBox/7e07cf8cc85e7f41 b/mp4/testdata/fuzz/FuzzDecodeBox/7e07cf8cc85e7f41
new file mode 100644
index 00000000..652d780c
--- /dev/null
+++ b/mp4/testdata/fuzz/FuzzDecodeBox/7e07cf8cc85e7f41
@@ -0,0 +1,2 @@
+go test fuzz v1
+[]byte("\x00\x00\x00\x10co640000 \x00\x00\x00")