diff --git a/contracts/Marketplace.sol b/contracts/Marketplace.sol
index 0f38754..d8c23ae 100644
--- a/contracts/Marketplace.sol
+++ b/contracts/Marketplace.sol
@@ -35,6 +35,7 @@ contract Marketplace is SlotReservations, Proofs, StateRetrieval, Endian {
   error Marketplace_InvalidState();
   error Marketplace_StartNotBeforeExpiry();
   error Marketplace_SlotNotAcceptingProofs();
+  error Marketplace_ProofNotSubmittedByHost();
   error Marketplace_SlotIsFree();
   error Marketplace_ReservationRequired();
   error Marketplace_NothingToWithdraw();
@@ -208,12 +209,12 @@ contract Marketplace is SlotReservations, Proofs, StateRetrieval, Endian {
       revert Marketplace_SlotNotFree();
     }
 
-    _startRequiringProofs(slotId);
-    submitProof(slotId, proof);
-
     slot.host = msg.sender;
     slot.filledAt = uint64(block.timestamp);
 
+    _startRequiringProofs(slotId);
+    submitProof(slotId, proof);
+
     context.slotsFilled += 1;
     context.fundsToReturnToClient -= _slotPayout(requestId, slot.filledAt);
 
@@ -321,6 +322,11 @@ contract Marketplace is SlotReservations, Proofs, StateRetrieval, Endian {
     Groth16Proof calldata proof
   ) public requestIsKnown(_slots[id].requestId) {
     Slot storage slot = _slots[id];
+
+    if (msg.sender != slot.host) {
+      revert Marketplace_ProofNotSubmittedByHost();
+    }
+
     Request storage request = _requests[slot.requestId];
     uint256[] memory pubSignals = new uint256[](3);
     pubSignals[0] = _challengeToFieldElement(getChallenge(id));
diff --git a/test/Marketplace.test.js b/test/Marketplace.test.js
index 073dcd4..f465c53 100644
--- a/test/Marketplace.test.js
+++ b/test/Marketplace.test.js
@@ -485,6 +485,13 @@ describe("Marketplace", function () {
       await marketplace.submitProof(slotId(slot), proof)
     })
 
+    it("reverts when somebody other then host submit the proof", async function () {
+      switchAccount(host2)
+      await expect(
+        marketplace.submitProof(slotId(slot), proof)
+      ).to.be.revertedWith("Marketplace_ProofNotSubmittedByHost")
+    })
+
     it("converts first 31 bytes of challenge to field element", async function () {
       let challenge = arrayify(await marketplace.getChallenge(slotId(slot)))
       let truncated = challenge.slice(0, 31)