diff --git a/src/sdk/account/utils/toWalletClient.test.ts b/src/sdk/account/utils/toWalletClient.test.ts
new file mode 100644
index 00000000..ed66bf02
--- /dev/null
+++ b/src/sdk/account/utils/toWalletClient.test.ts
@@ -0,0 +1,72 @@
+import { http, type Hex, createWalletClient } from "viem"
+import { privateKeyToAccount } from "viem/accounts"
+import { mainnet } from "viem/chains"
+import { afterEach, describe, expect, it } from "vitest"
+import { toSigner } from "./toSigner"
+import { toWalletClient } from "./toWalletClient"
+
+// Regression coverage for https://github.com/bcnmy/abstractjs/issues/212
+//
+// toWalletClient used to unconditionally build `custom(window?.ethereum)` when
+// the unresolved signer's transport key was "custom". When a user connects
+// without a wallet extension (e.g. a 3rd-party email / web2 connector),
+// `window.ethereum` is undefined, and `custom(undefined)` produced a transport
+// that threw "Cannot read properties of undefined (reading 'request')".
+describe("utils.toWalletClient", () => {
+  const account = privateKeyToAccount(
+    "0x1234567890123456789012345678901234567890123456789012345678901234" as Hex
+  )
+
+  afterEach(() => {
+    // Clean up any injected provider stub between cases.
+    if (typeof window !== "undefined") {
+      // biome-ignore lint/performance/noDelete: test cleanup
+      delete (window as { ethereum?: unknown }).ethereum
+    }
+  })
+
+  it("does not throw when the signer is 'custom' but no injected provider exists", async () => {
+    const resolvedSigner = await toSigner({ signer: account })
+    // A wallet client whose transport key is "custom" simulates a browser signer.
+    const unresolvedSigner = createWalletClient({
+      account,
+      chain: mainnet,
+      transport: http()
+    })
+    // Force the "custom" transport key that selects the browser branch, while
+    // leaving window.ethereum undefined (no extension installed).
+    ;(unresolvedSigner as { transport: { key: string } }).transport.key =
+      "custom"
+
+    expect(() =>
+      toWalletClient({
+        unresolvedSigner: unresolvedSigner as never,
+        resolvedSigner,
+        chain: mainnet,
+        transport: http()
+      })
+    ).not.toThrow()
+  })
+
+  it("falls back to the resolved signer account when there is no injected provider", async () => {
+    const resolvedSigner = await toSigner({ signer: account })
+    const unresolvedSigner = createWalletClient({
+      account,
+      chain: mainnet,
+      transport: http()
+    })
+    ;(unresolvedSigner as { transport: { key: string } }).transport.key =
+      "custom"
+
+    const walletClient = toWalletClient({
+      unresolvedSigner: unresolvedSigner as never,
+      resolvedSigner,
+      chain: mainnet,
+      transport: http()
+    })
+
+    // Without window.ethereum the account is the full local signer, not just an
+    // address routed through a (missing) injected provider.
+    expect(walletClient.account.address).toBe(account.address)
+  })
+})
diff --git a/src/sdk/account/utils/toWalletClient.ts b/src/sdk/account/utils/toWalletClient.ts
index b596542d..62b31c75 100644
--- a/src/sdk/account/utils/toWalletClient.ts
+++ b/src/sdk/account/utils/toWalletClient.ts
@@ -24,14 +24,27 @@ export const toWalletClient = ({
   chain,
   transport
 }: ToWalletClientParameters): ToWalletClientReturnType => {
-  const browserSigner = unresolvedSigner?.transport?.key === "custom"
+  // Only route through the injected browser provider when the signer was created
+  // from one (transport key "custom") AND an EIP-1193 provider is actually
+  // present on `window.ethereum`. When a user connects without a wallet
+  // extension (e.g. via a 3rd-party email / web2 connector), `window.ethereum`
+  // is undefined; `custom(undefined)` previously produced a transport that threw
+  // "Cannot read properties of undefined (reading 'request')" from
+  // toNexusAccount. In that case fall back to the resolved (local) signer and
+  // the provided transport. See https://github.com/bcnmy/abstractjs/issues/212
+  const injectedProvider =
+    typeof window !== "undefined"
+      ? (window as { ethereum?: unknown }).ethereum
+      : undefined
+  const useBrowserProvider =
+    unresolvedSigner?.transport?.key === "custom" && Boolean(injectedProvider)
   return createWalletClient(
-    browserSigner
+    useBrowserProvider
       ? {
           account: resolvedSigner.address,
           chain,
           // @ts-ignore
-          transport: custom(window?.ethereum)
+          transport: custom(injectedProvider)
         }
       : {
           account: resolvedSigner,