diff --git a/deploy/OneStepProverHostIoCreator.js b/deploy/OneStepProverHostIoCreator.js
index 8174c09c..f3df423d 100644
--- a/deploy/OneStepProverHostIoCreator.js
+++ b/deploy/OneStepProverHostIoCreator.js
@@ -5,9 +5,12 @@ module.exports = async hre => {
 
   await deploy('OneStepProverHostIo', {
     from: deployer,
-    args: [ethers.constants.AddressZero],
+    args: [
+      ethers.constants.AddressZero,
+      (await deployments.get('HashProofHelper')).address,
+    ],
   })
 }
 
 module.exports.tags = ['OneStepProverHostIo']
-module.exports.dependencies = []
+module.exports.dependencies = ['HashProofHelper']
diff --git a/src/osp/OneStepProofEntry.sol b/src/osp/OneStepProofEntry.sol
index 19253698..561c2aec 100644
--- a/src/osp/OneStepProofEntry.sol
+++ b/src/osp/OneStepProofEntry.sol
@@ -106,22 +106,28 @@ contract OneStepProofEntry is IOneStepProofEntry {
                 (globalState, offset) = Deserialize.globalState(proof, offset);
                 require(globalState.hash() == mach.globalStateHash, "BAD_GLOBAL_STATE");
 
-                MELState memory melState;
-                (melState, offset) = Deserialize.melState(proof, offset);
-                require(melState.hash() == globalState.getMELStateHash(), "BAD_MEL_STATE");
-
-                // The machine has finished processing a message and we're at the start of the next execution segment (machineStep == 0).
-                // If the MELState is not at its target (meaning that it hasn't finished extracting messages, which should only happen before the extraction process is started),
-                // or if all messages were extracted, but there are still messages to be executed in MEL, we kickstart the machine.
-                if (
-                    mach.status == MachineStatus.FINISHED && machineStep == 0
-                        && (
-                            melState.parentChainBlockHash != execCtx.targetParentChainBlockHash
-                                || globalState.getMELExecutedMsgCount() < melState.msgCount
-                        )
-                ) {
-                    // Kickstart the machine
-                    return getStartMachineHash(mach.globalStateHash, execCtx.initialWasmModuleRoot);
+                // Only MEL-enhanced proofs carry a MELState. For non-MEL halted
+                // machines (MELStateHash == 0) the prover serializes the global
+                // state and reads no further, so we must not attempt to read a
+                // MELState here either.
+                if (globalState.getMELStateHash() != bytes32(0)) {
+                    MELState memory melState;
+                    (melState, offset) = Deserialize.melState(proof, offset);
+                    require(melState.hash() == globalState.getMELStateHash(), "BAD_MEL_STATE");
+
+                    // The machine has finished processing a message and we're at the start of the next execution segment (machineStep == 0).
+                    // If the MELState is not at its target (meaning that it hasn't finished extracting messages, which should only happen before the extraction process is started),
+                    // or if all messages were extracted, but there are still messages to be executed in MEL, we kickstart the machine.
+                    if (
+                        mach.status == MachineStatus.FINISHED && machineStep == 0
+                            && (
+                                melState.parentChainBlockHash != execCtx.targetParentChainBlockHash
+                                    || globalState.getMELExecutedMsgCount() < melState.msgCount
+                            )
+                    ) {
+                        // Kickstart the machine
+                        return getStartMachineHash(mach.globalStateHash, execCtx.initialWasmModuleRoot);
+                    }
                 }
                 return mach.hash();
             }
diff --git a/test/prover/one-step-proof.ts b/test/prover/one-step-proof.ts
index 3ff5238d..14911e2d 100644
--- a/test/prover/one-step-proof.ts
+++ b/test/prover/one-step-proof.ts
@@ -74,12 +74,6 @@ describe('OneStepProof', function () {
           await deployments.get('OneStepProofEntry')
         ).address
       )
-      const bridge = await ethers.getContractAt(
-        'BridgeStub',
-        (
-          await deployments.get('BridgeStub')
-        ).address
-      )
 
       const promises = []
       const isdone = []
@@ -89,10 +83,9 @@ describe('OneStepProof', function () {
         )
         const proof = proofs[i]
         isdone.push(false)
-        const inboxLimit = 1000000
         const promise = osp
           .proveOneStep(
-            [inboxLimit, bridge.address],
+            [ethers.constants.HashZero, ethers.constants.HashZero],
             i,
             [...Buffer.from(proof.before, 'hex')],
             [...Buffer.from(proof.proof, 'hex')]