diff --git a/internal/internal_workflow_testsuite.go b/internal/internal_workflow_testsuite.go
index 87bf91379..bc00ae314 100644
--- a/internal/internal_workflow_testsuite.go
+++ b/internal/internal_workflow_testsuite.go
@@ -160,6 +160,7 @@ type (
 	updateResult struct {
 		success interface{}
 		err     error
+		mu      *sync.Mutex
 	}
 
 	// testWorkflowEnvironmentShared is the shared data between parent workflow and child workflow test environments
@@ -235,7 +236,8 @@ type (
 		queryHandler          func(string, *commonpb.Payloads, *commonpb.Header) (*commonpb.Payloads, error)
 		updateHandler         func(name string, id string, input *commonpb.Payloads, header *commonpb.Header, resp UpdateCallbacks)
 		updateMap             map[string]updateResult
-		startedHandler        func(r WorkflowExecution, e error)
+		// updateMapLock         map[string]sync.Mutex
+		startedHandler func(r WorkflowExecution, e error)
 
 		isWorkflowCompleted bool
 		testResult          converter.EncodedValue
@@ -2236,7 +2238,6 @@ func (env *testWorkflowEnvironmentImpl) RegisterUpdateHandler(
 	handler func(name string, id string, input *commonpb.Payloads, header *commonpb.Header, resp UpdateCallbacks),
 ) {
 	env.updateHandler = handler
-	env.updateMap = make(map[string]updateResult)
 }
 
 func (env *testWorkflowEnvironmentImpl) RegisterQueryHandler(
@@ -2925,19 +2926,28 @@ func (env *testWorkflowEnvironmentImpl) updateWorkflow(name string, id string, u
 		panic(err)
 	}
 
+	if env.updateMap == nil {
+		env.updateMap = make(map[string]updateResult)
+	}
+
 	var ucWrapper = updateCallbacksWrapper{uc: uc, env: env, updateID: id}
 
 	// check for duplicate update ID
 	if result, ok := env.updateMap[id]; ok {
+		result.mu.Lock()
 		// return cached result
 		env.postCallback(func() {
 			ucWrapper.uc.Accept()
 			ucWrapper.uc.Complete(result.success, result.err)
+			defer result.mu.Unlock()
 		}, false)
 	} else {
+		env.updateMap[id] = updateResult{nil, nil, &sync.Mutex{}}
+		env.updateMap[id].mu.Lock()
 		env.postCallback(func() {
 			// Do not send any headers on test invocations
 			env.updateHandler(name, id, data, nil, ucWrapper)
+			defer env.updateMap[id].mu.Unlock()
 		}, true)
 	}
 
@@ -3120,7 +3130,13 @@ func (uc updateCallbacksWrapper) Complete(success interface{}, err error) {
 	if uc.env == nil {
 		panic("env is needed in updateCallback to cache update results for deduping purposes")
 	}
-	uc.env.updateMap[uc.updateID] = updateResult{success, err}
+	if result, ok := uc.env.updateMap[uc.updateID]; ok {
+		result.success = success
+		result.err = err
+		uc.env.updateMap[uc.updateID] = result
+	} else {
+		panic("updateMap[updateID] should already be created from updateWorkflow()")
+	}
 	uc.uc.Complete(success, err)
 }