diff --git a/specs/phase0/beacon-chain.md b/specs/phase0/beacon-chain.md
index 8c93df7ef..88e85ac78 100644
--- a/specs/phase0/beacon-chain.md
+++ b/specs/phase0/beacon-chain.md
@@ -958,6 +958,7 @@ def get_beacon_proposer_index(state: BeaconState) -> ValidatorIndex:
 def get_total_balance(state: BeaconState, indices: Set[ValidatorIndex]) -> Gwei:
     """
     Return the combined effective balance of the ``indices``. (1 Gwei minimum to avoid divisions by zero.)
+    Math safe up to ~10B ETH, afterwhich this overflows uint64.
     """
     return Gwei(max(1, sum([state.validators[index].effective_balance for index in indices])))
 ```
@@ -1324,7 +1325,9 @@ 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 = EFFECTIVE_BALANCE_INCREMENT  # Factored out from balance totals to avoid uint64 overflow
+                reward_numerator = get_base_reward(state, index) * (attesting_balance // increment)
+                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,