diff --git a/shared/src/advent/challenges/day_three/repo.rs b/shared/src/advent/challenges/day_three/repo.rs
index 2c608b3..0a1901e 100644
--- a/shared/src/advent/challenges/day_three/repo.rs
+++ b/shared/src/advent/challenges/day_three/repo.rs
@@ -5,6 +5,16 @@ use atrium_crypto::keypair::P256Keypair;
 use atrium_repo::{Repository, blockstore::{CarStore, MemoryBlockStore}};
 use serde::Serialize;
 
+#[derive(Debug, thiserror::Error)]
+pub enum CarBuildError {
+    #[error("Signing error: {0}")]
+    Signing(#[from] atrium_crypto::Error),
+    #[error("Repo error: {0}")]
+    Repo(#[from] atrium_repo::repo::Error),
+    #[error("CAR error: {0}")]
+    Car(#[from] atrium_repo::blockstore::CarError),
+}
+
 /// secret record type
 #[derive(Debug, Serialize)]
 #[serde(rename_all = "camelCase")]
@@ -22,7 +32,7 @@ pub async fn manufacture_car(
     collection: &str,
     rkey: &str,
     record: ChallengeRecord,
-) -> Result<Vec<u8>, Box<dyn std::error::Error>> {
+) -> Result<Vec<u8>, CarBuildError> {
 
     // throwaway (fake key hello)
     let keypair = P256Keypair::import(&[1u8; 32])?;