diff --git a/src/arch/wasm32.rs b/src/arch/wasm32.rs
index e800f45..381ea96 100644
--- a/src/arch/wasm32.rs
+++ b/src/arch/wasm32.rs
@@ -4,7 +4,7 @@ use super::generic;
 use crate::get_chars_table;
 use core::arch::wasm32::*;
 
-pub(crate) const USE_CHECK_FN: bool = false;
+pub(crate) const USE_CHECK_FN: bool = true;
 
 #[inline(always)]
 fn has_simd128() -> bool {
diff --git a/src/lib.rs b/src/lib.rs
index e3918f7..ee712f2 100644
--- a/src/lib.rs
+++ b/src/lib.rs
@@ -609,6 +609,8 @@ fn decode_to_slice_inner(input: &[u8], output: &mut [u8]) -> Result<(), FromHexE
 /// Assumes `output.len() == input.len() / 2`.
 #[inline]
 unsafe fn decode_checked(input: &[u8], output: &mut [u8]) -> Result<(), FromHexError> {
+    debug_assert_eq!(output.len(), input.len() / 2);
+
     if imp::USE_CHECK_FN {
         // check then decode
         if imp::check(input) {
@@ -628,7 +630,7 @@ unsafe fn decode_checked(input: &[u8], output: &mut [u8]) -> Result<(), FromHexE
 #[inline]
 const fn byte2hex<const UPPER: bool>(byte: u8) -> (u8, u8) {
     let table = get_chars_table::<UPPER>();
-    let high = table[((byte & 0xf0) >> 4) as usize];
+    let high = table[(byte >> 4) as usize];
     let low = table[(byte & 0x0f) as usize];
     (high, low)
 }