diff --git a/codex/contracts/market.nim b/codex/contracts/market.nim
index 58495b45..0b846099 100644
--- a/codex/contracts/market.nim
+++ b/codex/contracts/market.nim
@@ -249,10 +249,16 @@ method fillSlot(
       requestId
       slotIndex
 
-    await market.approveFunds(collateral)
-    trace "calling fillSlot on contract"
-    discard await market.contract.fillSlot(requestId, slotIndex, proof).confirm(1)
-    trace "fillSlot transaction completed"
+    try:
+      await market.approveFunds(collateral)
+      trace "calling fillSlot on contract"
+      discard await market.contract.fillSlot(requestId, slotIndex, proof).confirm(1)
+      trace "fillSlot transaction completed"
+    except Marketplace_SlotNotFree as parent:
+      raise newException(
+        SlotStateMismatchError, "Failed to fill slot because the slot is not free",
+        parent,
+      )
 
 method freeSlot*(market: OnChainMarket, slotId: SlotId) {.async.} =
   convertEthersError("Failed to free slot"):
@@ -327,14 +333,20 @@ method reserveSlot*(
     market: OnChainMarket, requestId: RequestId, slotIndex: uint64
 ) {.async.} =
   convertEthersError("Failed to reserve slot"):
-    discard await market.contract
-    .reserveSlot(
-      requestId,
-      slotIndex,
-      # reserveSlot runs out of gas for unknown reason, but 100k gas covers it
-      TransactionOverrides(gasLimit: some 100000.u256),
-    )
-    .confirm(1)
+    try:
+      discard await market.contract
+      .reserveSlot(
+        requestId,
+        slotIndex,
+        # reserveSlot runs out of gas for unknown reason, but 100k gas covers it
+        TransactionOverrides(gasLimit: some 100000.u256),
+      )
+      .confirm(1)
+    except SlotReservations_ReservationNotAllowed:
+      raise newException(
+        SlotReservationNotAllowedError,
+        "Failed to reserve slot because reservation is not allowed",
+      )
 
 method canReserveSlot*(
     market: OnChainMarket, requestId: RequestId, slotIndex: uint64
diff --git a/codex/contracts/marketplace.nim b/codex/contracts/marketplace.nim
index 761caada..686414fb 100644
--- a/codex/contracts/marketplace.nim
+++ b/codex/contracts/marketplace.nim
@@ -53,6 +53,7 @@ type
   Proofs_ProofAlreadyMarkedMissing* = object of SolidityError
   Proofs_InvalidProbability* = object of SolidityError
   Periods_InvalidSecondsPerPeriod* = object of SolidityError
+  SlotReservations_ReservationNotAllowed* = object of SolidityError
 
 proc configuration*(marketplace: Marketplace): MarketplaceConfig {.contract, view.}
 proc token*(marketplace: Marketplace): Address {.contract, view.}
diff --git a/codex/market.nim b/codex/market.nim
index c5177aeb..dd8e14ba 100644
--- a/codex/market.nim
+++ b/codex/market.nim
@@ -18,6 +18,8 @@ export periods
 type
   Market* = ref object of RootObj
   MarketError* = object of CodexError
+  SlotStateMismatchError* = object of MarketError
+  SlotReservationNotAllowedError* = object of MarketError
   Subscription* = ref object of RootObj
   OnRequest* =
     proc(id: RequestId, ask: StorageAsk, expiry: uint64) {.gcsafe, upraises: [].}
diff --git a/codex/sales/states/filling.nim b/codex/sales/states/filling.nim
index 03e2ef2b..13644223 100644
--- a/codex/sales/states/filling.nim
+++ b/codex/sales/states/filling.nim
@@ -30,6 +30,7 @@ method run*(
 ): Future[?State] {.async: (raises: []).} =
   let data = SalesAgent(machine).data
   let market = SalesAgent(machine).context.market
+
   without (request =? data.request):
     raiseAssert "Request not set"
 
@@ -42,17 +43,16 @@ method run*(
       err:
       error "Failure attempting to fill slot: unable to calculate collateral",
         error = err.msg
-      return
+      return some State(SaleErrored(error: err))
 
     debug "Filling slot"
     try:
       await market.fillSlot(data.requestId, data.slotIndex, state.proof, collateral)
+    except SlotStateMismatchError as e:
+      debug "Slot is already filled, ignoring slot"
+      return some State(SaleIgnored(reprocessSlot: false, returnBytes: true))
     except MarketError as e:
-      if e.msg.contains "Slot is not free":
-        debug "Slot is already filled, ignoring slot"
-        return some State(SaleIgnored(reprocessSlot: false, returnBytes: true))
-      else:
-        return some State(SaleErrored(error: e))
+      return some State(SaleErrored(error: e))
     # other CatchableErrors are handled "automatically" by the SaleState
 
     return some State(SaleFilled())
diff --git a/codex/sales/states/slotreserving.nim b/codex/sales/states/slotreserving.nim
index a67c51a0..e9ac8dcd 100644
--- a/codex/sales/states/slotreserving.nim
+++ b/codex/sales/states/slotreserving.nim
@@ -44,12 +44,11 @@ method run*(
       try:
         trace "Reserving slot"
         await market.reserveSlot(data.requestId, data.slotIndex)
+      except SlotReservationNotAllowedError as e:
+        debug "Slot cannot be reserved, ignoring", error = e.msg
+        return some State(SaleIgnored(reprocessSlot: false, returnBytes: true))
       except MarketError as e:
-        if e.msg.contains "SlotReservations_ReservationNotAllowed":
-          debug "Slot cannot be reserved, ignoring", error = e.msg
-          return some State(SaleIgnored(reprocessSlot: false, returnBytes: true))
-        else:
-          return some State(SaleErrored(error: e))
+        return some State(SaleErrored(error: e))
       # other CatchableErrors are handled "automatically" by the SaleState
 
       trace "Slot successfully reserved"
diff --git a/tests/codex/helpers/mockmarket.nim b/tests/codex/helpers/mockmarket.nim
index 16806cb2..edf8a62d 100644
--- a/tests/codex/helpers/mockmarket.nim
+++ b/tests/codex/helpers/mockmarket.nim
@@ -46,7 +46,8 @@ type
     subscriptions: Subscriptions
     config*: MarketplaceConfig
     canReserveSlot*: bool
-    reserveSlotThrowError*: ?(ref MarketError)
+    errorOnReserveSlot*: ?(ref MarketError)
+    errorOnFillSlot*: ?(ref CatchableError)
     clock: ?Clock
 
   Fulfillment* = object
@@ -289,6 +290,9 @@ proc fillSlot*(
     host: Address,
     collateral = 0.u256,
 ) =
+  if error =? market.errorOnFillSlot:
+    raise error
+
   let slot = MockSlot(
     requestId: requestId,
     slotIndex: slotIndex,
@@ -370,7 +374,7 @@ method canProofBeMarkedAsMissing*(
 method reserveSlot*(
     market: MockMarket, requestId: RequestId, slotIndex: uint64
 ) {.async.} =
-  if error =? market.reserveSlotThrowError:
+  if error =? market.errorOnReserveSlot:
     raise error
 
 method canReserveSlot*(
@@ -381,8 +385,19 @@ method canReserveSlot*(
 func setCanReserveSlot*(market: MockMarket, canReserveSlot: bool) =
   market.canReserveSlot = canReserveSlot
 
-func setReserveSlotThrowError*(market: MockMarket, error: ?(ref MarketError)) =
-  market.reserveSlotThrowError = error
+func setErrorOnReserveSlot*(market: MockMarket, error: ref MarketError) =
+  market.errorOnReserveSlot =
+    if error.isNil:
+      none (ref MarketError)
+    else:
+      some error
+
+func setErrorOnFillSlot*(market: MockMarket, error: ref CatchableError) =
+  market.errorOnFillSlot =
+    if error.isNil:
+      none (ref CatchableError)
+    else:
+      some error
 
 method subscribeRequests*(
     market: MockMarket, callback: OnRequest
diff --git a/tests/codex/sales/states/testfilling.nim b/tests/codex/sales/states/testfilling.nim
index 1a26753d..f746b5a8 100644
--- a/tests/codex/sales/states/testfilling.nim
+++ b/tests/codex/sales/states/testfilling.nim
@@ -1,18 +1,31 @@
-import pkg/unittest2
 import pkg/questionable
 import pkg/codex/contracts/requests
 import pkg/codex/sales/states/filling
 import pkg/codex/sales/states/cancelled
 import pkg/codex/sales/states/failed
+import pkg/codex/sales/states/ignored
+import pkg/codex/sales/states/errored
+import pkg/codex/sales/salesagent
+import pkg/codex/sales/salescontext
+import ../../../asynctest
 import ../../examples
 import ../../helpers
+import ../../helpers/mockmarket
+import ../../helpers/mockclock
 
 suite "sales state 'filling'":
   let request = StorageRequest.example
   let slotIndex = request.ask.slots div 2
   var state: SaleFilling
+  var market: MockMarket
+  var clock: MockClock
+  var agent: SalesAgent
 
   setup:
+    clock = MockClock.new()
+    market = MockMarket.new()
+    let context = SalesContext(market: market, clock: clock)
+    agent = newSalesAgent(context, request.id, slotIndex, request.some)
     state = SaleFilling.new()
 
   test "switches to cancelled state when request expires":
@@ -22,3 +35,28 @@ suite "sales state 'filling'":
   test "switches to failed state when request fails":
     let next = state.onFailed(request)
     check !next of SaleFailed
+
+  test "run switches to ignored when slot is not free":
+    let error = newException(
+      SlotStateMismatchError, "Failed to fill slot because the slot is not free"
+    )
+    market.setErrorOnFillSlot(error)
+    market.requested.add(request)
+    market.slotState[request.slotId(slotIndex)] = SlotState.Filled
+
+    let next = !(await state.run(agent))
+    check next of SaleIgnored
+    check SaleIgnored(next).reprocessSlot == false
+    check SaleIgnored(next).returnBytes
+
+  test "run switches to errored with other error ":
+    let error = newException(MarketError, "some error")
+    market.setErrorOnFillSlot(error)
+    market.requested.add(request)
+    market.slotState[request.slotId(slotIndex)] = SlotState.Filled
+
+    let next = !(await state.run(agent))
+    check next of SaleErrored
+
+    let errored = SaleErrored(next)
+    check errored.error == error
diff --git a/tests/codex/sales/states/testslotreserving.nim b/tests/codex/sales/states/testslotreserving.nim
index d9ecdfc8..0e2e2cc7 100644
--- a/tests/codex/sales/states/testslotreserving.nim
+++ b/tests/codex/sales/states/testslotreserving.nim
@@ -54,15 +54,16 @@ asyncchecksuite "sales state 'SlotReserving'":
 
   test "run switches to errored when slot reservation errors":
     let error = newException(MarketError, "some error")
-    market.setReserveSlotThrowError(some error)
+    market.setErrorOnReserveSlot(error)
     let next = !(await state.run(agent))
     check next of SaleErrored
     let errored = SaleErrored(next)
     check errored.error == error
 
-  test "catches reservation not allowed error":
-    let error = newException(MarketError, "SlotReservations_ReservationNotAllowed")
-    market.setReserveSlotThrowError(some error)
+  test "run switches to ignored when reservation is not allowed":
+    let error =
+      newException(SlotReservationNotAllowedError, "Reservation is not allowed")
+    market.setErrorOnReserveSlot(error)
     let next = !(await state.run(agent))
     check next of SaleIgnored
     check SaleIgnored(next).reprocessSlot == false