diff --git a/bit-wrap/src/ast.rs b/bit-wrap/src/ast.rs
index 3ac5dfe..94bc441 100644
--- a/bit-wrap/src/ast.rs
+++ b/bit-wrap/src/ast.rs
@@ -2,33 +2,17 @@ use crate::grammar;
 use unsynn::*;
 
 pub struct Struct {
-    pub attr: Attribute,
+    pub bit_width: u128,
+    pub error_type: Ident,
     pub name: Ident,
     pub body: Ident,
 }
 
-impl Struct {
-    pub fn bit_width(&self) -> u128 {
-        self.attr.bit_width
-    }
-}
-
-pub struct Attribute {
-    pub bit_width: u128,
-}
-
-impl From<grammar::Attribute> for Attribute {
-    fn from(value: grammar::Attribute) -> Self {
-        Self {
-            bit_width: value.bit_width.content.bit_width.content.value(),
-        }
-    }
-}
-
 impl From<grammar::StructDef> for Struct {
     fn from(value: grammar::StructDef) -> Self {
         Self {
-            attr: value.attr.into(),
+            bit_width: value.bit_width.bit_width.content.width.content.value(),
+            error_type: value.error_type.error.content.error.content,
             name: value.name,
             body: value.body.content,
         }
diff --git a/bit-wrap/src/bit_wrapper.rs b/bit-wrap/src/bit_wrapper.rs
index 880e0e0..4eac0ee 100644
--- a/bit-wrap/src/bit_wrapper.rs
+++ b/bit-wrap/src/bit_wrapper.rs
@@ -2,6 +2,9 @@ use crate::{codegen::generate_impl, grammar::StructDef};
 use unsynn::*;
 
 pub fn impl_bit_wrapper(input: &TokenStream) -> TokenStream {
-    let parsed = input.to_token_iter().parse::<StructDef>().unwrap();
+    let parsed = input
+        .to_token_iter()
+        .parse::<StructDef>()
+        .expect("StructDef parsing");
     generate_impl(&parsed.into())
 }
diff --git a/bit-wrap/src/codegen.rs b/bit-wrap/src/codegen.rs
index 8f4bba9..f63c3d5 100644
--- a/bit-wrap/src/codegen.rs
+++ b/bit-wrap/src/codegen.rs
@@ -70,8 +70,9 @@ fn generate_bitwise_fmt(info: &Struct) -> TokenStream {
 fn generate_bitwise_ops(info: &Struct) -> TokenStream {
     let name = &info.name;
     let inner = &info.body;
+    let error_type = &info.error_type;
 
-    let bit_width = u8::try_from(info.bit_width()).expect("8-bit value");
+    let bit_width = u8::try_from(info.bit_width).expect("8-bit value");
 
     let hex_width = usize::from(bit_width.div_ceil(4));
     let bin_width = usize::from(bit_width);
@@ -91,7 +92,7 @@ fn generate_bitwise_ops(info: &Struct) -> TokenStream {
             /// Create a new [`Self`] from a key value
             #[inline]
             #[macro_use]
-            pub fn new(key: #inner) -> Result<Self, String> {
+            pub fn new(key: #inner) -> Result<Self, #error_type> {
                 key.try_into()
             }
 
@@ -263,17 +264,17 @@ fn generate_bitwise_ops(info: &Struct) -> TokenStream {
             }
 
             /// Create from a hex string with bit width validation
-            pub fn from_hex(hex: &str) -> Result<Self, String> {
-                let value = #inner::from_str_radix(hex, 16).map_err(|e| format!("Invalid hex string: {e}"))?;
+            pub fn from_hex(hex: &str) -> Result<Self, #error_type> {
+                let value = #inner::from_str_radix(hex, 16)?;
 
                 let masked = value & Self::MAX;
                 if value != masked {
-                    return Err(
-                        format!(
-                            "Hex value 0x{value:X} exceeds {}-bit limit (masked to 0x{masked:0width$X})",
-                            Self::BIT_WIDTH, width = #hex_width
-                        )
-                    )
+                    return Err(#error_type::ExceedsBitLimit {
+                        value,
+                        bit_width: Self::BIT_WIDTH,
+                        masked,
+                        width: #hex_width
+                    })
                 }
 
                 Ok(Self(value))
diff --git a/bit-wrap/src/grammar.rs b/bit-wrap/src/grammar.rs
index 9aa153a..c154790 100644
--- a/bit-wrap/src/grammar.rs
+++ b/bit-wrap/src/grammar.rs
@@ -4,12 +4,13 @@ keyword! {
     pub KwStruct = "struct";
     pub KwPub = "pub";
     pub KwBitWidth = "bit_width";
+    pub KwError = "bit_conversion_error";
 }
 
 unsynn! {
-    pub struct BitWidth{
+    pub struct BitWidth {
         pub kw_bit_width: KwBitWidth,
-        pub bit_width: ParenthesisGroupContaining<LiteralInteger>,
+        pub width: ParenthesisGroupContaining<LiteralInteger>,
     }
 
     pub struct Attribute {
@@ -17,8 +18,19 @@ unsynn! {
         pub bit_width: BracketGroupContaining<BitWidth>,
     }
 
+    pub struct ErrorType {
+        pub kw_bit_width: KwError,
+        pub error: ParenthesisGroupContaining<Ident>,
+    }
+
+    pub struct AttributeError {
+        pub pound: Pound,
+        pub error: BracketGroupContaining<ErrorType>,
+    }
+
     pub struct StructDef {
-        pub attr: Attribute,
+        pub bit_width: Attribute,
+        pub error_type: AttributeError,
         pub vis: Option<KwPub>,
         pub kw_struct: KwStruct,
         pub name: Ident,
@@ -33,6 +45,7 @@ mod tests {
 
     const SAMPLE: &str = r#"
         #[bit_width(48)]
+        #[bit_conversion_error()]
         pub struct Subkey(u64);
     "#;
 
diff --git a/bit-wrap/src/lib.rs b/bit-wrap/src/lib.rs
index ff30cb0..0921da9 100644
--- a/bit-wrap/src/lib.rs
+++ b/bit-wrap/src/lib.rs
@@ -6,7 +6,7 @@ mod grammar;
 use crate::bit_wrapper::impl_bit_wrapper;
 use proc_macro::TokenStream;
 
-#[proc_macro_derive(BitWrapper, attributes(bit_width))]
+#[proc_macro_derive(BitWrapper, attributes(bit_width, bit_conversion_error))]
 pub fn derive_bit_wrapper(input: TokenStream) -> TokenStream {
     impl_bit_wrapper(&input.into()).into()
 }