diff --git a/specs/core/0_beacon-chain.md b/specs/core/0_beacon-chain.md
index f69795926..60f774e9a 100644
--- a/specs/core/0_beacon-chain.md
+++ b/specs/core/0_beacon-chain.md
@@ -51,6 +51,7 @@
         - [`hash`](#hash)
         - [`hash_tree_root`](#hash_tree_root)
         - [`signing_root`](#signing_root)
+        - [`bls_domain`](#bls_domain)
         - [`slot_to_epoch`](#slot_to_epoch)
         - [`get_previous_epoch`](#get_previous_epoch)
         - [`get_current_epoch`](#get_current_epoch)
@@ -629,6 +630,16 @@ The `hash` function is SHA256.
 
 `def signing_root(object: SSZContainer) -> Bytes32` is a function defined in the [SimpleSerialize spec](../simple-serialize.md#self-signed-containers) to compute signing messages.
 
+### `bls_domain`
+
+```python
+def bls_domain(domain_type: int, fork_version: bytes=b'\x00\x00\x00\x00') -> int:
+    """
+    Return the bls domain given by the ``domain_type`` and optional 4 byte ``fork_version`` (defaults to zero).
+    """
+    return bytes_to_int(int_to_bytes(domain_type, length=4) + fork_version)
+```
+
 ### `slot_to_epoch`
 
 ```python
@@ -968,7 +979,7 @@ def get_domain(state: BeaconState,
     """
     epoch = get_current_epoch(state) if message_epoch is None else message_epoch
     fork_version = state.fork.previous_version if epoch < state.fork.epoch else state.fork.current_version
-    return bytes_to_int(fork_version + int_to_bytes(domain_type, length=4))
+    return bls_domain(domain_type, fork_version)
 ```
 
 ### `get_bitfield_bit`
@@ -1765,8 +1776,9 @@ def process_deposit(state: BeaconState, deposit: Deposit) -> None:
     validator_pubkeys = [v.pubkey for v in state.validator_registry]
     if pubkey not in validator_pubkeys:
         # Verify the deposit signature (proof of possession)
+        # Note: deposits are valid across forks, hence the deposit domain is retrieved directly from `bls_domain`
         if not bls_verify(
-            pubkey, signing_root(deposit.data), deposit.data.signature, get_domain(state, DOMAIN_DEPOSIT)
+            pubkey, signing_root(deposit.data), deposit.data.signature, bls_domain(DOMAIN_DEPOSIT)
         ):
             return
 
diff --git a/specs/validator/0_beacon-chain-validator.md b/specs/validator/0_beacon-chain-validator.md
index d05f25ef2..f8272d446 100644
--- a/specs/validator/0_beacon-chain-validator.md
+++ b/specs/validator/0_beacon-chain-validator.md
@@ -98,7 +98,7 @@ To submit a deposit:
 * Pack the validator's [initialization parameters](#initialization) into `deposit_data`, a [`DepositData`](../core/0_beacon-chain.md#depositdata) SSZ object.
 * Let `amount` be the amount in Gwei to be deposited by the validator where `MIN_DEPOSIT_AMOUNT <= amount <= MAX_EFFECTIVE_BALANCE`.
 * Set `deposit_data.amount = amount`.
-* Let `signature` be the result of `bls_sign` of the `signing_root(deposit_data)` with `domain=DOMAIN_DEPOSIT`.
+* Let `signature` be the result of `bls_sign` of the `signing_root(deposit_data)` with `domain=bls_domain(DOMAIN_DEPOSIT)`. (Deposits are valid regardless of fork version, `bls_domain` will default to zeroes there).
 * Send a transaction on the Ethereum 1.0 chain to `DEPOSIT_CONTRACT_ADDRESS` executing `def deposit(pubkey: bytes[48], withdrawal_credentials: bytes[32], signature: bytes[96])` along with a deposit of `amount` Gwei.
 
 *Note*: Deposits made for the same `pubkey` are treated as for the same validator. A singular `Validator` will be added to `state.validator_registry` with each additional deposit amount added to the validator's balance. A validator can only be activated when total deposits for the validator pubkey meet or exceed `MAX_EFFECTIVE_BALANCE`.
diff --git a/test_libs/pyspec/tests/helpers.py b/test_libs/pyspec/tests/helpers.py
index 7af210f85..a4849bfbb 100644
--- a/test_libs/pyspec/tests/helpers.py
+++ b/test_libs/pyspec/tests/helpers.py
@@ -24,6 +24,7 @@ from eth2spec.phase0.spec import (
     VoluntaryExit,
     # functions
     convert_to_indexed,
+    bls_domain,
     get_active_validator_indices,
     get_attesting_indices,
     get_block_root,
@@ -144,10 +145,7 @@ def build_deposit_data(state, pubkey, privkey, amount):
     signature = bls.sign(
         message_hash=signing_root(deposit_data),
         privkey=privkey,
-        domain=get_domain(
-            state,
-            spec.DOMAIN_DEPOSIT,
-        )
+        domain=bls_domain(spec.DOMAIN_DEPOSIT),
     )
     deposit_data.signature = signature
     return deposit_data