diff --git a/beacon_chain/beacon_node.nim b/beacon_chain/beacon_node.nim
index 5d40d14ce..aba4d1631 100644
--- a/beacon_chain/beacon_node.nim
+++ b/beacon_chain/beacon_node.nim
@@ -281,6 +281,12 @@ proc makeAttestation(node: BeaconNode,
                      shard: uint64,
                      committeeLen: int,
                      indexInCommittee: int) {.async.} =
+                     
+  # TODO - move that to "updateState"
+  # Epoch underflow - https://github.com/status-im/nim-beacon-chain/issues/207
+  doAssert node.state.data.current_justified_epoch != GENESIS_EPOCH - 1,
+    "Underflow in justified epoch field before making attestation"
+
   let
     attestationData =
       makeAttestationData(node.state.data, shard, node.state.blck.root)
diff --git a/beacon_chain/spec/validator.nim b/beacon_chain/spec/validator.nim
index b00bb7c72..1ee4cac46 100644
--- a/beacon_chain/spec/validator.nim
+++ b/beacon_chain/spec/validator.nim
@@ -125,7 +125,11 @@ func get_next_epoch_committee_count(state: BeaconState): uint64 =
 # https://github.com/ethereum/eth2.0-specs/blob/v0.5.0/specs/core/0_beacon-chain.md#get_previous_epoch
 func get_previous_epoch*(state: BeaconState): Epoch =
   ## Return the previous epoch of the given ``state``.
-  get_current_epoch(state) - 1
+  let epoch = get_current_epoch(state)
+  if epoch == GENESIS_EPOCH:
+    raise newException(OverflowError, "Underflow: trying to get the previous epoch of epoch 0 (genesis).")
+  epoch - 1
+
 
 # https://github.com/ethereum/eth2.0-specs/blob/v0.5.0/specs/core/0_beacon-chain.md#get_crosslink_committees_at_slot
 func get_crosslink_committees_at_slot*(state: BeaconState, slot: Slot|uint64,