diff --git a/specs/phase0/beacon-chain.md b/specs/phase0/beacon-chain.md
index acf098663..7c14550bb 100644
--- a/specs/phase0/beacon-chain.md
+++ b/specs/phase0/beacon-chain.md
@@ -194,6 +194,7 @@ The following values are (non-configurable) constants used throughout the specif
 | `MAX_EFFECTIVE_BALANCE` | `Gwei(2**5 * 10**9)` (= 32,000,000,000) |
 | `EJECTION_BALANCE` | `Gwei(2**4 * 10**9)` (= 16,000,000,000) |
 | `EFFECTIVE_BALANCE_INCREMENT` | `Gwei(2**0 * 10**9)` (= 1,000,000,000) |
+| `REWARD_OVERFLOW_INCREMENT` | `Gwei(2**6)` (= 64) |
 
 ### Initial values
 
@@ -1313,7 +1314,9 @@ def get_attestation_deltas(state: BeaconState) -> Tuple[Sequence[Gwei], Sequence
         attesting_balance = get_total_balance(state, unslashed_attesting_indices)
         for index in eligible_validator_indices:
             if index in unslashed_attesting_indices:
-                rewards[index] += get_base_reward(state, index) * attesting_balance // total_balance
+                increment = REWARD_OVERFLOW_INCREMENT  # Factored out from reward numerator to avoid uint64 overflow
+                reward_numerator = get_base_reward(state, index) // increment * attesting_balance
+                rewards[index] = reward_numerator // total_balance * increment
             else:
                 penalties[index] += get_base_reward(state, index)
 
diff --git a/tests/core/pyspec/eth2spec/test/phase_0/epoch_processing/test_process_rewards_and_penalties.py b/tests/core/pyspec/eth2spec/test/phase_0/epoch_processing/test_process_rewards_and_penalties.py
index fa394df56..b4f50179e 100644
--- a/tests/core/pyspec/eth2spec/test/phase_0/epoch_processing/test_process_rewards_and_penalties.py
+++ b/tests/core/pyspec/eth2spec/test/phase_0/epoch_processing/test_process_rewards_and_penalties.py
@@ -1,7 +1,10 @@
 from copy import deepcopy
 
-from eth2spec.test.context import spec_state_test, with_all_phases, spec_test, \
-    misc_balances, with_custom_state, default_activation_threshold, single_phase
+from eth2spec.test.context import (
+    spec_state_test, with_all_phases, spec_test,
+    misc_balances, with_custom_state, default_activation_threshold,
+    single_phase,
+)
 from eth2spec.test.helpers.state import (
     next_epoch,
     next_slot,