diff --git a/.changeset/calm-cats-sign.md b/.changeset/calm-cats-sign.md
new file mode 100644
index 00000000..858d85ae
--- /dev/null
+++ b/.changeset/calm-cats-sign.md
@@ -0,0 +1,5 @@
+---
+"@zerodev/weighted-validator": patch
+---
+
+Handle missing initial UserOperation signatures and replace the current signer's stub without dropping co-signer signatures.
diff --git a/plugins/weighted-r1-k1/toWeightedValidatorPlugin.test.ts b/plugins/weighted-r1-k1/toWeightedValidatorPlugin.test.ts
new file mode 100644
index 00000000..540c4e9c
--- /dev/null
+++ b/plugins/weighted-r1-k1/toWeightedValidatorPlugin.test.ts
@@ -0,0 +1,92 @@
+import { describe, expect, test } from "bun:test"
+import { createClient, custom } from "viem"
+import { privateKeyToAccount } from "viem/accounts"
+import { mainnet } from "viem/chains"
+import {
+    WeightedValidatorContractVersion,
+    createWeightedValidator
+} from "./toWeightedValidatorPlugin.js"
+import { decodeSignatures, encodeSignatures } from "./utils.js"
+
+const owner = privateKeyToAccount(
+    "0x2827b876ee775816460ab6eb4481352a752101f950899831702ccead54000001"
+)
+
+const entryPointAddress = "0x0000000071727De22E5E9d8BAf0edAc6f37da032"
+
+const getValidator = async () => {
+    const signer = {
+        account: owner,
+        getPublicKey: () => owner.address,
+        getDummySignature: () => `0x${"00".repeat(65)}` as `0x${string}`,
+        type: "0x01" as never
+    }
+    const client = createClient({
+        chain: mainnet,
+        transport: custom({
+            request: async ({ method }) => {
+                if (method === "eth_chainId") return "0x1"
+                throw new Error(`Unexpected RPC method: ${method}`)
+            }
+        })
+    })
+
+    return createWeightedValidator(client, {
+        entryPoint: {
+            address: entryPointAddress,
+            version: "0.7"
+        },
+        kernelVersion: "0.3.1",
+        validatorContractVersion:
+            WeightedValidatorContractVersion.V0_0_2_PATCHED,
+        signer,
+        config: {
+            threshold: 1,
+            signers: [{ publicKey: owner.address, weight: 1 }]
+        }
+    })
+}
+
+const userOperation = {
+    sender: "0x0000000000000000000000000000000000000001",
+    nonce: 0n,
+    callData: "0x",
+    callGasLimit: 100_000n,
+    verificationGasLimit: 100_000n,
+    preVerificationGas: 50_000n,
+    maxFeePerGas: 1n,
+    maxPriorityFeePerGas: 1n,
+    signature: "0x"
+} as const
+
+describe("createWeightedValidator", () => {
+    test("creates a stub signature when the UserOperation signature is undefined", async () => {
+        const validator = await getValidator()
+
+        const signature = await validator.getStubSignature({
+            ...userOperation,
+            signature: undefined
+        } as never)
+
+        expect(decodeSignatures(signature)).toHaveLength(1)
+    })
+
+    test("replaces its stub while preserving co-signer signatures", async () => {
+        const validator = await getValidator()
+        const cosignerSignature = `0x01${"11".repeat(65)}` as `0x${string}`
+        const stubSignature = await validator.getStubSignature({
+            ...userOperation,
+            signature: encodeSignatures([cosignerSignature])
+        })
+
+        const signature = await validator.signUserOperation({
+            ...userOperation,
+            signature: stubSignature
+        })
+
+        const signatures = decodeSignatures(signature)
+        expect(signatures).toHaveLength(2)
+        expect(signatures[0]).toBe(cosignerSignature)
+        expect(signatures[1]).not.toBe(decodeSignatures(stubSignature)[1])
+    })
+})
diff --git a/plugins/weighted-r1-k1/toWeightedValidatorPlugin.ts b/plugins/weighted-r1-k1/toWeightedValidatorPlugin.ts
index 2d407608..66035b36 100644
--- a/plugins/weighted-r1-k1/toWeightedValidatorPlugin.ts
+++ b/plugins/weighted-r1-k1/toWeightedValidatorPlugin.ts
@@ -207,8 +207,16 @@ export async function createWeightedValidator<
         // Sign a user operation
         async signUserOperation(userOperation) {
             let signatures: readonly Hex[] = []
-            if (userOperation.signature !== "0x") {
-                signatures = decodeSignatures(userOperation.signature)
+            if (userOperation.signature && userOperation.signature !== "0x") {
+                const ownStubSignature = concatHex([
+                    toHex(getIndexOfSigner(), { size: 1 }),
+                    await signer.getDummySignature()
+                ])
+                signatures = decodeSignatures(userOperation.signature).filter(
+                    (signature) =>
+                        signature.toLowerCase() !==
+                        ownStubSignature.toLowerCase()
+                )
             }
             // last signer signs for userOpHash
             const userOpHash = getUserOperationHash({
@@ -230,8 +238,7 @@ export async function createWeightedValidator<
 
         async getStubSignature(userOperation) {
             let signatures: readonly Hex[] = []
-            if (userOperation.signature !== "0x") {
-                console.log(userOperation.signature)
+            if (userOperation.signature && userOperation.signature !== "0x") {
                 signatures = decodeSignatures(userOperation.signature)
             }