diff --git a/temporalio/testing/_workflow.py b/temporalio/testing/_workflow.py
index b63e0b57..3015a86d 100644
--- a/temporalio/testing/_workflow.py
+++ b/temporalio/testing/_workflow.py
@@ -562,5 +562,5 @@ def _client_with_interceptors(
     config = client.config()
     config_interceptors = list(config["interceptors"])
     config_interceptors.extend(interceptors)
-    config["interceptors"] = interceptors
+    config["interceptors"] = config_interceptors
     return temporalio.client.Client(**config)
diff --git a/tests/testing/test_workflow.py b/tests/testing/test_workflow.py
index 2dc0eb8c..e0c1117b 100644
--- a/tests/testing/test_workflow.py
+++ b/tests/testing/test_workflow.py
@@ -3,12 +3,19 @@
 import uuid
 from datetime import datetime, timedelta, timezone
 from time import monotonic
-from typing import Optional, Union
+from typing import Any, List, Optional, Union
 
 import pytest
 
 from temporalio import activity, workflow
-from temporalio.client import Client, WorkflowFailureError
+from temporalio.client import (
+    Client,
+    Interceptor,
+    OutboundInterceptor,
+    StartWorkflowInput,
+    WorkflowFailureError,
+    WorkflowHandle,
+)
 from temporalio.common import RetryPolicy
 from temporalio.exceptions import (
     ActivityError,
@@ -176,7 +183,36 @@ def some_signal(self) -> None:
         assert "foo" == "bar"
 
 
+class SimpleClientInterceptor(Interceptor):
+    def __init__(self) -> None:
+        self.events: List[str] = []
+
+    def intercept_client(self, next: OutboundInterceptor) -> OutboundInterceptor:
+        return SimpleClientOutboundInterceptor(self, super().intercept_client(next))
+
+
+class SimpleClientOutboundInterceptor(OutboundInterceptor):
+    def __init__(
+        self, root: SimpleClientInterceptor, next: OutboundInterceptor
+    ) -> None:
+        super().__init__(next)
+        self.root = root
+
+    async def start_workflow(
+        self, input: StartWorkflowInput
+    ) -> WorkflowHandle[Any, Any]:
+        self.root.events.append(f"start: {input.workflow}")
+        return await super().start_workflow(input)
+
+
 async def test_workflow_env_assert(client: Client):
+    # Set the interceptor on the client. This used to fail for being
+    # accidentally overridden.
+    client_config = client.config()
+    interceptor = SimpleClientInterceptor()
+    client_config["interceptors"] = [interceptor]
+    client = Client(**client_config)
+
     def assert_proper_error(err: Optional[BaseException]) -> None:
         assert isinstance(err, ApplicationError)
         # In unsandboxed workflows, this message has extra diff info appended
@@ -195,6 +231,7 @@ def assert_proper_error(err: Optional[BaseException]) -> None:
                     task_queue=worker.task_queue,
                 )
             assert_proper_error(err.value.cause)
+            assert interceptor.events
 
             # Start a new one and check signal
             handle = await env.client.start_workflow(