diff --git a/varint-rs/src/lib.rs b/varint-rs/src/lib.rs
index 4a0a4855..7c660bf6 100644
--- a/varint-rs/src/lib.rs
+++ b/varint-rs/src/lib.rs
@@ -123,34 +123,48 @@ pub trait EncoderHelper: Sized {
 /// Helper trait to allow multiple integer types to be encoded
 pub trait DecoderHelper: Sized {
     /// Decode a single byte
-    fn decode_one(decoder: &mut DecoderState<Self>, byte: u8) -> Option<Self>;
+    fn decode_one(decoder: &mut DecoderState<Self>, byte: u8) -> errors::Result<Option<Self>>;
 
     /// Read as much of the varint as possible
     fn read<R: AsyncRead>(decoder: &mut DecoderState<Self>, input: R) -> Poll<Option<Self>, Error>;
 }
 
 macro_rules! impl_decoderstate {
-    ($t:ty) => { impl_decoderstate!($t, <$t>::from, |v| v); };
+    ($t:ty) => {
+        impl_decoderstate!(
+            $t,
+            |a| a as $t,
+            |a: $t, b| -> Option<$t> { a.checked_shl(b as u32) }
+        );
+    };
     ($t:ty, $make_fn:expr) => { impl_decoderstate!($t, $make_fn, $make_fn); };
-    ($t:ty, $make_fn:expr, $make_shift_fn:expr) => {
+    ($t:ty, $make_fn:expr, $shift_fn:expr) => {
         impl DecoderHelper for $t {
             #[inline]
-            fn decode_one(decoder: &mut DecoderState<Self>, byte: u8) -> Option<$t> {
-                decoder.accumulator.take().and_then(|accumulator| {
-                    let out = accumulator |
-                    (
-                        $make_fn(byte & 0x7F) <<
-                            $make_shift_fn(decoder.shift * USABLE_BITS_PER_BYTE)
-                    );
+            fn decode_one(decoder: &mut DecoderState<Self>, byte: u8) -> ::errors::Result<Option<$t>> {
+                let res = decoder.accumulator.take().and_then(|accumulator| {
+                    let out = accumulator | match $shift_fn(
+                        $make_fn(byte & 0x7F),
+                        decoder.shift * USABLE_BITS_PER_BYTE,
+                    ) {
+                        Some(a) => a,
+                        None => return Some(Err(ErrorKind::ParseError.into())),
+                    };
                     decoder.shift += 1;
 
                     if byte & 0x80 == 0 {
-                        Some(out)
+                        Some(Ok(out))
                     } else {
                         decoder.accumulator = AccumulatorState::InProgress(out);
                         None
                     }
-                })
+                });
+
+                match res {
+                    Some(Ok(number)) => Ok(Some(number)),
+                    Some(Err(err)) => Err(err),
+                    None => Ok(None),
+                }
             }
 
             fn read<R: AsyncRead>(
@@ -173,7 +187,7 @@ macro_rules! impl_decoderstate {
 
                     match input.read_exact(&mut buffer) {
                         Ok(()) => {
-                            if let Some(out) = Self::decode_one(decoder, buffer[0]) {
+                            if let Some(out) = Self::decode_one(decoder, buffer[0])? {
                                 break Ok(Async::Ready(Some(out)));
                             }
                         }
@@ -258,9 +272,9 @@ impl_encoderstate!(u64, (|val| val as u64));
 impl_encoderstate!(u32, (|val| val as u32));
 
 impl_decoderstate!(usize);
-impl_decoderstate!(BigUint);
-impl_decoderstate!(u64, (|val| val as u64));
-impl_decoderstate!(u32, (|val| val as u32));
+impl_decoderstate!(BigUint, BigUint::from, |a, b| Some(a << b));
+impl_decoderstate!(u64);
+impl_decoderstate!(u32);
 
 impl<T> EncoderState<T> {
     pub fn source(&self) -> &T {
@@ -368,7 +382,8 @@ impl<T: Default + DecoderHelper> Decoder for VarintDecoder<T> {
                 // We know that the length is not 0, so this cannot fail.
                 let first_byte = src.split_to(1)[0];
                 let mut state = self.state.take().unwrap_or_default();
-                let out = T::decode_one(&mut state, first_byte);
+                let out = T::decode_one(&mut state, first_byte)
+                    .map_err(|_| io::Error::from(io::ErrorKind::Other))?;
 
                 if let Some(out) = out {
                     break Ok(Some(out));
@@ -390,10 +405,12 @@ pub fn decode<R: Read, T: Default + DecoderHelper>(mut input: R) -> errors::Resu
 
         match input.read_exact(&mut buffer) {
             Ok(()) => {
-                            if let Some(out) = T::decode_one(&mut decoder, buffer[0]) {
-                                break Ok(out);
-                            }
-                        }
+                if let Some(out) = T::decode_one(&mut decoder, buffer[0])
+                    .map_err(|_| io::Error::from(io::ErrorKind::Other))?
+                {
+                    break Ok(out);
+                }
+            }
             Err(inner) => break Err(Error::with_chain(inner, ErrorKind::ParseError)),
         }
     }
@@ -417,6 +434,34 @@ mod tests {
     use num_bigint::BigUint;
     use futures::{Future, Stream};
 
+    #[test]
+    fn large_number_fails() {
+        use std::io::Cursor;
+        use futures::Async;
+        use super::WriteState;
+
+        let mut out = vec![0u8; 10];
+
+        {
+            let writable: Cursor<&mut [_]> = Cursor::new(&mut out);
+
+            let mut state = EncoderState::new(::std::u64::MAX);
+
+            assert_eq!(
+                state.write(writable).unwrap(),
+                Async::Ready(WriteState::Done(10))
+            );
+        }
+
+        let result: Result<Option<u32>, _> = FramedRead::new(&out[..], VarintDecoder::new())
+            .into_future()
+            .map(|(out, _)| out)
+            .map_err(|(out, _)| out)
+            .wait();
+
+        assert!(result.is_err());
+    }
+
     #[test]
     fn can_decode_basic_biguint() {
         assert_eq!(