diff --git a/rust/src/pcube/mod.rs b/rust/src/pcube/mod.rs
index c96bc25..1c4f9b8 100644
--- a/rust/src/pcube/mod.rs
+++ b/rust/src/pcube/mod.rs
@@ -52,8 +52,6 @@ where
     }
 }
 
-const MAX_PREFILL: u64 = 0x7FFF_FFFF_FFFF_FFFF;
-
 impl<T> PCubeFile<T>
 where
     T: Read,
@@ -170,14 +168,28 @@ impl PCubeFile {
 
             let [next_byte] = next_byte;
 
-            cube_count |= ((next_byte & 0x7F) as u64) << shift;
+            let value = (next_byte & 0x7F) as u64;
 
-            shift += 7;
-            if shift > 64 {
-                panic!("Cannot load possibly more than u64 cubes...");
+            let is_last_byte = next_byte & 0x80 == 0;
+            let is_single_bit = next_byte & 0x1 == 1 || next_byte == 0;
+            let overshift = shift > 56;
+
+            if shift > 63
+                || overshift && !is_last_byte && !is_single_bit
+                || overshift && !is_single_bit
+            {
+                return Err(std::io::Error::new(
+                    std::io::ErrorKind::InvalidData,
+                    "Cannot load more than u64 cubes",
+                ));
             }
 
-            if next_byte & 0x80 == 0 {
+            cube_count |= value.overflowing_shl(shift).0;
+            shift += 7;
+
+            println!("{:08X}", cube_count);
+
+            if is_last_byte {
                 break;
             }
         }
@@ -188,27 +200,16 @@ impl PCubeFile {
     /// Write a leb128 value
     ///
     /// If `prefill` is `true`, this function will always
-    /// write 9 bytes of data describing `number`.
-    ///
-    /// This way, one can rewrite the data after it is known how many cubes
-    /// are present.
-    ///
-    /// # Panics
-    /// This function panics if `prefill && number > MAX_PREFILL`
-    // TODO: this should probably take a `u128`?
+    /// write 10 bytes of data describing `number`.
     fn write_leb128(mut number: u64, mut writer: impl Write, prefill: bool) -> std::io::Result<()> {
-        if prefill && number > MAX_PREFILL {
-            panic!("Cannot prefill LEB128 value longer than 9 bytes");
-        }
-
         let mut ran_once = false;
         let mut bytes_written = 0;
-        while number > 0 || !ran_once || (prefill && bytes_written < 9) {
+        while number > 0 || !ran_once || (prefill && bytes_written < 10) {
             ran_once = true;
             let mut next_byte = (number as u8) & 0x7F;
             number >>= 7;
 
-            if number > 0 || (prefill && bytes_written != 8) {
+            if number > 0 || (prefill && bytes_written != 9) {
                 next_byte |= 0x80;
             }
 
@@ -222,11 +223,7 @@ impl PCubeFile {
     /// Write the header
     ///
     /// If `prefill_len` is `true`, the length is _always_ written
-    /// as 9 bytes. This way, rewriting the header in-place is possible.
-    ///
-    /// # Panics
-    /// This function panics if `prefill && number > MAX_PREFILL`
-    // TODO: this should probably take a `u128`?
+    /// as 10 bytes. This way, rewriting the header in-place is possible.
     fn write_header(
         mut write: impl Write,
         magic: [u8; 4],
@@ -304,11 +301,7 @@ impl PCubeFile {
         Self::write_header(&mut seekable, magic, is_canonical, compression, len, true)?;
 
         let len = Self::write_impl(cubes, compression, &mut seekable)?;
-        let len = if len as u64 <= MAX_PREFILL {
-            Some(len as u64)
-        } else {
-            None
-        };
+        let len = Some(len as u64);
 
         // Write magic and cube length at the end
         seekable.rewind()?;
@@ -475,19 +468,27 @@ where
 impl<T> AllUniquePolycubeIterator for AllUnique<T> where T: Read {}
 
 #[test]
-pub fn len() {
-    let values = [0, 1, 24, 150283, 0x7FFFF_FFFF, MAX_PREFILL];
+pub fn leb128_len() {
+    let values = [0, 1, 24, 150283, 0x7FFFF_FFFF, u64::MAX - 1, u64::MAX];
 
     for value in values {
         let mut data = Vec::new();
         PCubeFile::write_leb128(value, &mut data, true).unwrap();
+
         assert_eq!(value, PCubeFile::read_leb128(&data[..]).unwrap());
     }
 }
 
-#[should_panic]
 #[test]
-pub fn len_panics() {
-    let mut data = Vec::new();
-    PCubeFile::write_leb128(MAX_PREFILL + 1, &mut data, true).ok();
+pub fn leb128_unparseable() {
+    let unparseable_values = [
+        &[0x81, 0x80, 0x80, 0x80, 0x80, 0x80, 0x80, 0x80, 0x80, 0x02][..],
+        &[
+            0x81, 0x80, 0x80, 0x80, 0x80, 0x80, 0x80, 0x80, 0x80, 0x80, 0x01,
+        ][..],
+    ];
+
+    for unparseable in unparseable_values {
+        assert!(PCubeFile::read_leb128(unparseable).is_err());
+    }
 }