diff --git a/lib/clif-backend/src/signal/unix.rs b/lib/clif-backend/src/signal/unix.rs
index feaf4e2e2..3e57a1588 100644
--- a/lib/clif-backend/src/signal/unix.rs
+++ b/lib/clif-backend/src/signal/unix.rs
@@ -76,7 +76,7 @@ pub fn call_protected<T>(handler_data: &HandlerData, f: impl FnOnce() -> T) -> R
             *jmp_buf = prev_jmp_buf;
 
             if let Some(data) = super::TRAP_EARLY_DATA.with(|cell| cell.replace(None)) {
-                Err(RuntimeError::Panic { data })
+                Err(RuntimeError::Error { data })
             } else {
                 let (faulting_addr, inst_ptr) = CAUGHT_ADDRESSES.with(|cell| cell.get());
 
diff --git a/lib/emscripten/src/varargs.rs b/lib/emscripten/src/varargs.rs
index 3775d102c..a6a1fe46b 100644
--- a/lib/emscripten/src/varargs.rs
+++ b/lib/emscripten/src/varargs.rs
@@ -1,8 +1,5 @@
 use std::mem;
-use wasmer_runtime_core::{
-    types::{Type, WasmExternType},
-    vm::Ctx,
-};
+use wasmer_runtime_core::{types::WasmExternType, vm::Ctx};
 
 #[repr(transparent)]
 #[derive(Copy, Clone)]
@@ -19,12 +16,12 @@ impl VarArgs {
 }
 
 unsafe impl WasmExternType for VarArgs {
-    const TYPE: Type = Type::I32;
+    type Native = i32;
 
-    fn to_bits(self) -> u64 {
-        self.pointer as u64
+    fn to_native(self) -> Self::Native {
+        self.pointer as _
     }
-    fn from_bits(n: u64) -> Self {
+    fn from_native(n: Self::Native) -> Self {
         Self { pointer: n as u32 }
     }
 }
diff --git a/lib/runtime-core/src/error.rs b/lib/runtime-core/src/error.rs
index 090214834..84ea04bc2 100644
--- a/lib/runtime-core/src/error.rs
+++ b/lib/runtime-core/src/error.rs
@@ -1,4 +1,4 @@
-use crate::types::{FuncSig, GlobalDescriptor, MemoryDescriptor, TableDescriptor, Type, Value};
+use crate::types::{FuncSig, GlobalDescriptor, MemoryDescriptor, TableDescriptor, Type};
 use core::borrow::Borrow;
 use std::any::Any;
 
@@ -121,8 +121,7 @@ impl std::error::Error for LinkError {}
 /// Comparing two `RuntimeError`s always evaluates to false.
 pub enum RuntimeError {
     Trap { msg: Box<str> },
-    Exception { data: Box<[Value]> },
-    Panic { data: Box<dyn Any> },
+    Error { data: Box<dyn Any> },
 }
 
 impl PartialEq for RuntimeError {
@@ -137,10 +136,7 @@ impl std::fmt::Display for RuntimeError {
             RuntimeError::Trap { ref msg } => {
                 write!(f, "WebAssembly trap occured during runtime: {}", msg)
             }
-            RuntimeError::Exception { ref data } => {
-                write!(f, "Uncaught WebAssembly exception: {:?}", data)
-            }
-            RuntimeError::Panic { data } => {
+            RuntimeError::Error { data } => {
                 let msg = if let Some(s) = data.downcast_ref::<String>() {
                     s
                 } else if let Some(s) = data.downcast_ref::<&str>() {
diff --git a/lib/runtime-core/src/typed_func.rs b/lib/runtime-core/src/typed_func.rs
index 5f14a8fa1..ef1b3c077 100644
--- a/lib/runtime-core/src/typed_func.rs
+++ b/lib/runtime-core/src/typed_func.rs
@@ -7,6 +7,7 @@ use crate::{
 };
 use std::{
     any::Any,
+    convert::Infallible,
     ffi::c_void,
     fmt,
     marker::PhantomData,
@@ -120,14 +121,16 @@ pub trait TrapEarly<Rets>
 where
     Rets: WasmTypeList,
 {
-    fn report(self) -> Result<Rets, Box<dyn Any>>;
+    type Error: 'static;
+    fn report(self) -> Result<Rets, Self::Error>;
 }
 
 impl<Rets> TrapEarly<Rets> for Rets
 where
     Rets: WasmTypeList,
 {
-    fn report(self) -> Result<Rets, Box<dyn Any>> {
+    type Error = Infallible;
+    fn report(self) -> Result<Rets, Infallible> {
         Ok(self)
     }
 }
@@ -135,10 +138,11 @@ where
 impl<Rets, E> TrapEarly<Rets> for Result<Rets, E>
 where
     Rets: WasmTypeList,
-    E: Into<Box<Any>> + 'static,
+    E: 'static,
 {
-    fn report(self) -> Result<Rets, Box<dyn Any>> {
-        self.map_err(|err| err.into())
+    type Error = E;
+    fn report(self) -> Result<Rets, E> {
+        self
     }
 }
 
@@ -329,11 +333,13 @@ macro_rules! impl_traits {
                     let f: FN = unsafe { mem::transmute_copy(&()) };
 
                     let err = match panic::catch_unwind(panic::AssertUnwindSafe(|| {
-                        let res = f( ctx $( ,WasmExternType::from_native($x) )* ).report();
-                        res
+                        f( ctx $( ,WasmExternType::from_native($x) )* ).report()
                     })) {
                         Ok(Ok(returns)) => return returns.into_c_struct(),
-                        Ok(Err(err)) => err,
+                        Ok(Err(err)) => {
+                            let b: Box<_> = err.into();
+                            b as Box<dyn Any>
+                        },
                         Err(err) => err,
                     };
 
diff --git a/lib/wasi/src/lib.rs b/lib/wasi/src/lib.rs
index c08428dbe..82de5ad1f 100644
--- a/lib/wasi/src/lib.rs
+++ b/lib/wasi/src/lib.rs
@@ -17,6 +17,12 @@ pub use self::utils::is_wasi_module;
 
 use wasmer_runtime_core::{func, import::ImportObject, imports};
 
+/// This is returned in the Box<dyn Any> RuntimeError::Error variant.
+/// Use `downcast` or `downcast_ref` to retrieve the `ExitCode`.
+pub struct ExitCode {
+    pub code: syscalls::types::__wasi_exitcode_t,
+}
+
 pub fn generate_import_object(
     args: Vec<Vec<u8>>,
     envs: Vec<Vec<u8>>,
diff --git a/lib/wasi/src/ptr.rs b/lib/wasi/src/ptr.rs
index 85a582df0..55a16d40c 100644
--- a/lib/wasi/src/ptr.rs
+++ b/lib/wasi/src/ptr.rs
@@ -2,7 +2,7 @@ use crate::syscalls::types::{__wasi_errno_t, __WASI_EFAULT};
 use std::{cell::Cell, fmt, marker::PhantomData, mem};
 use wasmer_runtime_core::{
     memory::Memory,
-    types::{Type, ValueType, WasmExternType},
+    types::{ValueType, WasmExternType},
 };
 
 pub struct Array;
@@ -73,12 +73,12 @@ impl<T: Copy + ValueType> WasmPtr<T, Array> {
 }
 
 unsafe impl<T: Copy, Ty> WasmExternType for WasmPtr<T, Ty> {
-    const TYPE: Type = Type::I32;
+    type Native = i32;
 
-    fn to_bits(self) -> u64 {
-        self.offset as u64
+    fn to_native(self) -> Self::Native {
+        self.offset as i32
     }
-    fn from_bits(n: u64) -> Self {
+    fn from_native(n: Self::Native) -> Self {
         Self {
             offset: n as u32,
             _phantom: PhantomData,
diff --git a/lib/wasi/src/syscalls/mod.rs b/lib/wasi/src/syscalls/mod.rs
index a436444fb..ba9d66b5d 100644
--- a/lib/wasi/src/syscalls/mod.rs
+++ b/lib/wasi/src/syscalls/mod.rs
@@ -9,6 +9,7 @@ use self::types::*;
 use crate::{
     ptr::{Array, WasmPtr},
     state::{Fd, InodeVal, Kind, WasiFile, WasiState, MAX_SYMLINKS},
+    ExitCode,
 };
 use rand::{thread_rng, Rng};
 use std::cell::Cell;
@@ -1430,9 +1431,9 @@ pub fn poll_oneoff(
     debug!("wasi::poll_oneoff");
     unimplemented!()
 }
-pub fn proc_exit(ctx: &mut Ctx, rval: __wasi_exitcode_t) -> Result<(), &'static str> {
+pub fn proc_exit(ctx: &mut Ctx, code: __wasi_exitcode_t) -> Result<(), ExitCode> {
     debug!("wasi::proc_exit, {}", rval);
-    Err("Instance exited")
+    Err(ExitCode { code })
 }
 pub fn proc_raise(ctx: &mut Ctx, sig: __wasi_signal_t) -> __wasi_errno_t {
     debug!("wasi::proc_raise");
diff --git a/lib/wasi/src/syscalls/types.rs b/lib/wasi/src/syscalls/types.rs
index c030df312..c239031a4 100644
--- a/lib/wasi/src/syscalls/types.rs
+++ b/lib/wasi/src/syscalls/types.rs
@@ -43,84 +43,84 @@ pub struct __wasi_dirent_t {
     pub d_type: __wasi_filetype_t,
 }
 
-pub type __wasi_errno_t = u32;
-pub const __WASI_ESUCCESS: u32 = 0;
-pub const __WASI_E2BIG: u32 = 1;
-pub const __WASI_EACCES: u32 = 2;
-pub const __WASI_EADDRINUSE: u32 = 3;
-pub const __WASI_EADDRNOTAVAIL: u32 = 4;
-pub const __WASI_EAFNOSUPPORT: u32 = 5;
-pub const __WASI_EAGAIN: u32 = 6;
-pub const __WASI_EALREADY: u32 = 7;
-pub const __WASI_EBADF: u32 = 8;
-pub const __WASI_EBADMSG: u32 = 9;
-pub const __WASI_EBUSY: u32 = 10;
-pub const __WASI_ECANCELED: u32 = 11;
-pub const __WASI_ECHILD: u32 = 12;
-pub const __WASI_ECONNABORTED: u32 = 13;
-pub const __WASI_ECONNREFUSED: u32 = 14;
-pub const __WASI_ECONNRESET: u32 = 15;
-pub const __WASI_EDEADLK: u32 = 16;
-pub const __WASI_EDESTADDRREQ: u32 = 17;
-pub const __WASI_EDOM: u32 = 18;
-pub const __WASI_EDQUOT: u32 = 19;
-pub const __WASI_EEXIST: u32 = 20;
-pub const __WASI_EFAULT: u32 = 21;
-pub const __WASI_EFBIG: u32 = 22;
-pub const __WASI_EHOSTUNREACH: u32 = 23;
-pub const __WASI_EIDRM: u32 = 24;
-pub const __WASI_EILSEQ: u32 = 25;
-pub const __WASI_EINPROGRESS: u32 = 26;
-pub const __WASI_EINTR: u32 = 27;
-pub const __WASI_EINVAL: u32 = 28;
-pub const __WASI_EIO: u32 = 29;
-pub const __WASI_EISCONN: u32 = 30;
-pub const __WASI_EISDIR: u32 = 31;
-pub const __WASI_ELOOP: u32 = 32;
-pub const __WASI_EMFILE: u32 = 33;
-pub const __WASI_EMLINK: u32 = 34;
-pub const __WASI_EMSGSIZE: u32 = 35;
-pub const __WASI_EMULTIHOP: u32 = 36;
-pub const __WASI_ENAMETOOLONG: u32 = 37;
-pub const __WASI_ENETDOWN: u32 = 38;
-pub const __WASI_ENETRESET: u32 = 39;
-pub const __WASI_ENETUNREACH: u32 = 40;
-pub const __WASI_ENFILE: u32 = 41;
-pub const __WASI_ENOBUFS: u32 = 42;
-pub const __WASI_ENODEV: u32 = 43;
-pub const __WASI_ENOENT: u32 = 44;
-pub const __WASI_ENOEXEC: u32 = 45;
-pub const __WASI_ENOLCK: u32 = 46;
-pub const __WASI_ENOLINK: u32 = 47;
-pub const __WASI_ENOMEM: u32 = 48;
-pub const __WASI_ENOMSG: u32 = 49;
-pub const __WASI_ENOPROTOOPT: u32 = 50;
-pub const __WASI_ENOSPC: u32 = 51;
-pub const __WASI_ENOSYS: u32 = 52;
-pub const __WASI_ENOTCONN: u32 = 53;
-pub const __WASI_ENOTDIR: u32 = 54;
-pub const __WASI_ENOTEMPTY: u32 = 55;
-pub const __WASI_ENOTRECOVERABLE: u32 = 56;
-pub const __WASI_ENOTSOCK: u32 = 57;
-pub const __WASI_ENOTSUP: u32 = 58;
-pub const __WASI_ENOTTY: u32 = 59;
-pub const __WASI_ENXIO: u32 = 60;
-pub const __WASI_EOVERFLOW: u32 = 61;
-pub const __WASI_EOWNERDEAD: u32 = 62;
-pub const __WASI_EPERM: u32 = 63;
-pub const __WASI_EPIPE: u32 = 64;
-pub const __WASI_EPROTO: u32 = 65;
-pub const __WASI_EPROTONOSUPPORT: u32 = 66;
-pub const __WASI_EPROTOTYPE: u32 = 67;
-pub const __WASI_ERANGE: u32 = 68;
-pub const __WASI_EROFS: u32 = 69;
-pub const __WASI_ESPIPE: u32 = 70;
-pub const __WASI_ESRCH: u32 = 71;
-pub const __WASI_ESTALE: u32 = 72;
-pub const __WASI_ETIMEDOUT: u32 = 73;
-pub const __WASI_ETXTBSY: u32 = 74;
-pub const __WASI_EXDEV: u32 = 75;
-pub const __WASI_ENOTCAPABLE: u32 = 76;
+pub type __wasi_errno_t = u16;
+pub const __WASI_ESUCCESS: u16 = 0;
+pub const __WASI_E2BIG: u16 = 1;
+pub const __WASI_EACCES: u16 = 2;
+pub const __WASI_EADDRINUSE: u16 = 3;
+pub const __WASI_EADDRNOTAVAIL: u16 = 4;
+pub const __WASI_EAFNOSUPPORT: u16 = 5;
+pub const __WASI_EAGAIN: u16 = 6;
+pub const __WASI_EALREADY: u16 = 7;
+pub const __WASI_EBADF: u16 = 8;
+pub const __WASI_EBADMSG: u16 = 9;
+pub const __WASI_EBUSY: u16 = 10;
+pub const __WASI_ECANCELED: u16 = 11;
+pub const __WASI_ECHILD: u16 = 12;
+pub const __WASI_ECONNABORTED: u16 = 13;
+pub const __WASI_ECONNREFUSED: u16 = 14;
+pub const __WASI_ECONNRESET: u16 = 15;
+pub const __WASI_EDEADLK: u16 = 16;
+pub const __WASI_EDESTADDRREQ: u16 = 17;
+pub const __WASI_EDOM: u16 = 18;
+pub const __WASI_EDQUOT: u16 = 19;
+pub const __WASI_EEXIST: u16 = 20;
+pub const __WASI_EFAULT: u16 = 21;
+pub const __WASI_EFBIG: u16 = 22;
+pub const __WASI_EHOSTUNREACH: u16 = 23;
+pub const __WASI_EIDRM: u16 = 24;
+pub const __WASI_EILSEQ: u16 = 25;
+pub const __WASI_EINPROGRESS: u16 = 26;
+pub const __WASI_EINTR: u16 = 27;
+pub const __WASI_EINVAL: u16 = 28;
+pub const __WASI_EIO: u16 = 29;
+pub const __WASI_EISCONN: u16 = 30;
+pub const __WASI_EISDIR: u16 = 31;
+pub const __WASI_ELOOP: u16 = 32;
+pub const __WASI_EMFILE: u16 = 33;
+pub const __WASI_EMLINK: u16 = 34;
+pub const __WASI_EMSGSIZE: u16 = 35;
+pub const __WASI_EMULTIHOP: u16 = 36;
+pub const __WASI_ENAMETOOLONG: u16 = 37;
+pub const __WASI_ENETDOWN: u16 = 38;
+pub const __WASI_ENETRESET: u16 = 39;
+pub const __WASI_ENETUNREACH: u16 = 40;
+pub const __WASI_ENFILE: u16 = 41;
+pub const __WASI_ENOBUFS: u16 = 42;
+pub const __WASI_ENODEV: u16 = 43;
+pub const __WASI_ENOENT: u16 = 44;
+pub const __WASI_ENOEXEC: u16 = 45;
+pub const __WASI_ENOLCK: u16 = 46;
+pub const __WASI_ENOLINK: u16 = 47;
+pub const __WASI_ENOMEM: u16 = 48;
+pub const __WASI_ENOMSG: u16 = 49;
+pub const __WASI_ENOPROTOOPT: u16 = 50;
+pub const __WASI_ENOSPC: u16 = 51;
+pub const __WASI_ENOSYS: u16 = 52;
+pub const __WASI_ENOTCONN: u16 = 53;
+pub const __WASI_ENOTDIR: u16 = 54;
+pub const __WASI_ENOTEMPTY: u16 = 55;
+pub const __WASI_ENOTRECOVERABLE: u16 = 56;
+pub const __WASI_ENOTSOCK: u16 = 57;
+pub const __WASI_ENOTSUP: u16 = 58;
+pub const __WASI_ENOTTY: u16 = 59;
+pub const __WASI_ENXIO: u16 = 60;
+pub const __WASI_EOVERFLOW: u16 = 61;
+pub const __WASI_EOWNERDEAD: u16 = 62;
+pub const __WASI_EPERM: u16 = 63;
+pub const __WASI_EPIPE: u16 = 64;
+pub const __WASI_EPROTO: u16 = 65;
+pub const __WASI_EPROTONOSUPPORT: u16 = 66;
+pub const __WASI_EPROTOTYPE: u16 = 67;
+pub const __WASI_ERANGE: u16 = 68;
+pub const __WASI_EROFS: u16 = 69;
+pub const __WASI_ESPIPE: u16 = 70;
+pub const __WASI_ESRCH: u16 = 71;
+pub const __WASI_ESTALE: u16 = 72;
+pub const __WASI_ETIMEDOUT: u16 = 73;
+pub const __WASI_ETXTBSY: u16 = 74;
+pub const __WASI_EXDEV: u16 = 75;
+pub const __WASI_ENOTCAPABLE: u16 = 76;
 
 #[derive(Debug, Copy, Clone, PartialEq, Eq)]
 #[repr(C)]