diff --git a/beacon_node/beacon_chain/src/attestation_production.rs b/beacon_node/beacon_chain/src/attestation_production.rs
index febd2ff9a..40284877e 100644
--- a/beacon_node/beacon_chain/src/attestation_production.rs
+++ b/beacon_node/beacon_chain/src/attestation_production.rs
@@ -3,9 +3,10 @@ use types::{AttestationData, Hash256};
 
 #[derive(Debug, PartialEq)]
 pub enum Error {
-    SlotTooOld,
-    PresentSlotUnknown,
-    StateError,
+    /// The `justified_block_root` is unknown. This is an internal error.
+    UnknownJustifiedRoot,
+    /// The `epoch_boundary_root` is unknown. This is an internal error.
+    UnknownBoundaryRoot,
 }
 
 impl<T, U> BeaconChain<T, U>
@@ -13,13 +14,14 @@ where
     T: ClientDB,
     U: SlotClock,
 {
+    /// Produce an `AttestationData` that is valid for the present `slot` and given `shard`.
     pub fn produce_attestation_data(&self, shard: u64) -> Result<AttestationData, Error> {
         let justified_slot = self.justified_slot();
         let justified_block_root = self
             .state
             .read()
             .get_block_root(justified_slot, &self.spec)
-            .ok_or_else(|| Error::SlotTooOld)?
+            .ok_or_else(|| Error::UnknownJustifiedRoot)?
             .clone();
 
         let epoch_boundary_root = self
@@ -29,7 +31,7 @@ where
                 self.state.read().current_epoch_start_slot(&self.spec),
                 &self.spec,
             )
-            .ok_or_else(|| Error::SlotTooOld)?
+            .ok_or_else(|| Error::UnknownBoundaryRoot)?
             .clone();
 
         Ok(AttestationData {