diff --git a/src/bbox.rs b/src/bbox.rs
index bb0aa9d..ba95113 100644
--- a/src/bbox.rs
+++ b/src/bbox.rs
@@ -155,7 +155,11 @@ impl<'a> BoxHolder<'a> {
     pub fn parse(input: &'a [u8]) -> IResult<&'a [u8], BoxHolder<'a>> {
         let (_, header) = BoxHeader::parse(input)?;
         tracing::debug!(box_type = header.box_type, ?header, "Got");
-        let (remain, data) = streaming::take(header.box_size)(input)?;
+
+        let box_size = usize::try_from(header.box_size)
+            .expect("header box size should always fit into a `usize`.");
+
+        let (remain, data) = streaming::take(box_size)(input)?;
 
         Ok((remain, BoxHolder { header, data }))
     }
diff --git a/src/bbox/idat.rs b/src/bbox/idat.rs
index 88bf6e6..1534ba7 100644
--- a/src/bbox/idat.rs
+++ b/src/bbox/idat.rs
@@ -14,7 +14,10 @@ pub struct IdatBox<'a> {
 impl<'a> IdatBox<'a> {
     pub fn parse(input: &'a [u8]) -> IResult<&'a [u8], IdatBox> {
         let (remain, header) = BoxHeader::parse(input)?;
-        let (remain, data) = streaming::take(header.box_size - header.header_size as u64)(remain)?;
+
+        let box_size = usize::try_from(header.box_size).expect("box size must fit into a `usize`.");
+
+        let (remain, data) = streaming::take(box_size - header.header_size)(remain)?;
 
         Ok((remain, IdatBox { header, data }))
     }