diff --git a/internal/internal_workflow_testsuite.go b/internal/internal_workflow_testsuite.go
index 21b785fc6..43f36e669 100644
--- a/internal/internal_workflow_testsuite.go
+++ b/internal/internal_workflow_testsuite.go
@@ -2452,13 +2452,26 @@ func (env *testWorkflowEnvironmentImpl) RequestCancelChildWorkflow(_, workflowID
 
 func (env *testWorkflowEnvironmentImpl) RequestCancelExternalWorkflow(namespace, workflowID, runID string, callback ResultHandler) {
 	if env.workflowInfo.WorkflowExecution.ID == workflowID {
-		// cancel current workflow
-		env.workflowCancelHandler()
-		// check if current workflow is a child workflow
-		if env.isChildWorkflow() && env.onChildWorkflowCanceledListener != nil {
-			env.postCallback(func() {
-				env.onChildWorkflowCanceledListener(env.workflowInfo)
-			}, false)
+		cancelFunc := func() {
+			env.workflowCancelHandler()
+
+			if env.isChildWorkflow() && env.onChildWorkflowCanceledListener != nil {
+				env.postCallback(func() {
+					env.onChildWorkflowCanceledListener(env.workflowInfo)
+				}, false)
+			}
+		}
+		// The way testWorkflowEnvironment is setup today, we close the child workflow dispatcher before calling
+		// the workflowCancelHandler. A larger refactor would be needed to handle this similar to non-test code.
+		// Maybe worth doing when https://github.com/temporalio/go-sdk/issues/50 is tackled.
+		if sd, ok := env.workflowDef.(*syncWorkflowDefinition); ok && env.isChildWorkflow() {
+			if !sd.dispatcher.IsClosed() {
+				sd.dispatcher.NewCoroutine(sd.rootCtx, "cancel-self", true, func(ctx Context) {
+					cancelFunc()
+				})
+			}
+		} else {
+			cancelFunc()
 		}
 		return
 	} else if childHandle, ok := env.runningWorkflows[workflowID]; ok && !childHandle.handled {
diff --git a/internal/workflow_testsuite_test.go b/internal/workflow_testsuite_test.go
index 2f8af03bc..f2b7b9152 100644
--- a/internal/workflow_testsuite_test.go
+++ b/internal/workflow_testsuite_test.go
@@ -1234,6 +1234,43 @@ func TestDynamicWorkflows(t *testing.T) {
 	require.Equal(t, "dynamic-activity - grape - cherry", result)
 }
 
+func SleepHour(ctx Context) error {
+	// We need to specifically have a timer that's cancelled, so that the
+	// timer's underlying channel is closed when the workflow is cancelled
+	return Sleep(ctx, time.Hour)
+}
+
+func SleepThenCancel(ctx Context) error {
+	cwf := ExecuteChildWorkflow(
+		ctx,
+		SleepHour,
+	)
+	var res WorkflowExecution
+	if err := cwf.GetChildWorkflowExecution().Get(ctx, &res); err != nil {
+		return err
+	}
+
+	err := RequestCancelExternalWorkflow(ctx, res.ID, res.RunID).Get(ctx, nil)
+	if err != nil {
+		return err
+	}
+
+	// This Sleep yields control back to startMainLoop, allowing it to process the
+	// child's cancel callback. Without it, the parent completes before the cancel
+	// propagates. The cancel callback then calls workflowCancelHandler,
+	// which would previously cause a panic.
+	return Sleep(ctx, time.Second)
+}
+
+func TestRequestCancelExternalWorkflowInSelector(t *testing.T) {
+	testSuite := &WorkflowTestSuite{}
+	env := testSuite.NewTestWorkflowEnvironment()
+	env.RegisterWorkflow(SleepHour)
+	env.ExecuteWorkflow(SleepThenCancel)
+	require.NoError(t, env.GetWorkflowError())
+	env.IsWorkflowCompleted()
+}
+
 func checkActivityInfo(ctx context.Context, isWorkflowActivity bool) error {
 	info := GetActivityInfo(ctx)
 	if isWorkflowActivity {