diff --git a/contracts/Timestamps.sol b/contracts/Timestamps.sol
index f751e05..6e2dcda 100644
--- a/contracts/Timestamps.sol
+++ b/contracts/Timestamps.sol
@@ -3,19 +3,24 @@ pragma solidity 0.8.28;
 
 type Timestamp is uint64;
 
-using {_notEquals as !=} for Timestamp global;
-using {_lessThan as <} for Timestamp global;
-using {_atMost as <=} for Timestamp global;
+using {_timestampEquals as ==} for Timestamp global;
+using {_timestampNotEqual as !=} for Timestamp global;
+using {_timestampLessThan as <} for Timestamp global;
+using {_timestampAtMost as <=} for Timestamp global;
 
-function _notEquals(Timestamp a, Timestamp b) pure returns (bool) {
+function _timestampEquals(Timestamp a, Timestamp b) pure returns (bool) {
+  return Timestamp.unwrap(a) == Timestamp.unwrap(b);
+}
+
+function _timestampNotEqual(Timestamp a, Timestamp b) pure returns (bool) {
   return Timestamp.unwrap(a) != Timestamp.unwrap(b);
 }
 
-function _lessThan(Timestamp a, Timestamp b) pure returns (bool) {
+function _timestampLessThan(Timestamp a, Timestamp b) pure returns (bool) {
   return Timestamp.unwrap(a) < Timestamp.unwrap(b);
 }
 
-function _atMost(Timestamp a, Timestamp b) pure returns (bool) {
+function _timestampAtMost(Timestamp a, Timestamp b) pure returns (bool) {
   return Timestamp.unwrap(a) <= Timestamp.unwrap(b);
 }
 
diff --git a/contracts/TokensPerSecond.sol b/contracts/TokensPerSecond.sol
index 677da3a..455a66e 100644
--- a/contracts/TokensPerSecond.sol
+++ b/contracts/TokensPerSecond.sol
@@ -5,13 +5,18 @@ import "./Timestamps.sol";
 
 type TokensPerSecond is int256;
 
-using {_negate as -} for TokensPerSecond global;
-using {_equals as ==} for TokensPerSecond global;
+using {_tokensPerSecondNegate as -} for TokensPerSecond global;
+using {_tokensPerSecondEquals as ==} for TokensPerSecond global;
 
-function _negate(TokensPerSecond rate) pure returns (TokensPerSecond) {
+function _tokensPerSecondNegate(
+  TokensPerSecond rate
+) pure returns (TokensPerSecond) {
   return TokensPerSecond.wrap(-TokensPerSecond.unwrap(rate));
 }
 
-function _equals(TokensPerSecond a, TokensPerSecond b) pure returns (bool) {
+function _tokensPerSecondEquals(
+  TokensPerSecond a,
+  TokensPerSecond b
+) pure returns (bool) {
   return TokensPerSecond.unwrap(a) == TokensPerSecond.unwrap(b);
 }
diff --git a/contracts/VaultBase.sol b/contracts/VaultBase.sol
index d3b8fad..5aee717 100644
--- a/contracts/VaultBase.sol
+++ b/contracts/VaultBase.sol
@@ -157,10 +157,8 @@ abstract contract VaultBase {
     Timestamp expiry,
     Timestamp maximum
   ) internal {
-    require(
-      Timestamp.unwrap(_getLock(controller, context).maximum) == 0,
-      AlreadyLocked()
-    );
+    Lock memory existing = _getLock(controller, context);
+    require(existing.maximum == Timestamp.wrap(0), AlreadyLocked());
     require(expiry <= maximum, ExpiryPastMaximum());
     _locks[controller][context] = Lock({expiry: expiry, maximum: maximum});
   }
@@ -170,10 +168,10 @@ abstract contract VaultBase {
     Context context,
     Timestamp expiry
   ) internal {
-    Lock memory previous = _getLock(controller, context);
-    require(Timestamps.currentTime() < previous.expiry, LockExpired());
-    require(previous.expiry <= expiry, InvalidExpiry());
-    require(expiry <= previous.maximum, ExpiryPastMaximum());
+    Lock memory existing = _getLock(controller, context);
+    require(Timestamps.currentTime() < existing.expiry, LockExpired());
+    require(existing.expiry <= expiry, InvalidExpiry());
+    require(expiry <= existing.maximum, ExpiryPastMaximum());
     _locks[controller][context].expiry = expiry;
   }
 
@@ -185,8 +183,9 @@ abstract contract VaultBase {
     TokensPerSecond rate
   ) internal {
     Lock memory lock = _getLock(controller, context);
-    require(lock.expiry != Timestamp.wrap(0), LockRequired());
     Timestamp start = Timestamps.currentTime();
+    require(lock.expiry != Timestamp.wrap(0), LockRequired());
+    require(start < lock.expiry, LockExpired());
     uint64 duration = Timestamp.unwrap(lock.maximum) - Timestamp.unwrap(start);
     int256 total = int256(uint256(duration)) * TokensPerSecond.unwrap(rate);
     Balance memory balance = _getBalance(controller, context, from);
diff --git a/test/Vault.tests.js b/test/Vault.tests.js
index 74da3c7..ca58a6d 100644
--- a/test/Vault.tests.js
+++ b/test/Vault.tests.js
@@ -406,6 +406,15 @@ describe("Vault", function () {
       )
     })
 
+    it("requires that the lock is not expired", async function () {
+      let expiry = (await currentTime()) + 20
+      await vault.lockup(context, expiry, expiry)
+      await advanceTimeTo(expiry)
+      await expect(vault.flow(context, sender, receiver, 2)).to.be.revertedWith(
+        "LockExpired"
+      )
+    })
+
     describe("when a lock is set", async function () {
       let expiry
       let maximum