diff --git a/CHANGELOG.next.asciidoc b/CHANGELOG.next.asciidoc
index a10ceba2da8..cc505b4ad8f 100644
--- a/CHANGELOG.next.asciidoc
+++ b/CHANGELOG.next.asciidoc
@@ -149,6 +149,7 @@ automatic splitting at root level, if root level element is an array. {pull}3415
 - Fix project monitor temp directories permission to include group access. {pull}35398[35398]
 - Fix output pipeline exit on run_once. {pull}35376[35376]
 - Fix formatting issue with socket trace timeout. {pull}35434[35434]
+- Fix serialization of processors when running diagnostics. {pull}35698[35698]
 
 *Heartbeat*
 
diff --git a/libbeat/publisher/processing/default.go b/libbeat/publisher/processing/default.go
index a8ec007562e..e3a2c961a92 100644
--- a/libbeat/publisher/processing/default.go
+++ b/libbeat/publisher/processing/default.go
@@ -261,9 +261,13 @@ func newBuilder(
 // Processors returns a string description of the processor config
 func (b *builder) Processors() []string {
 	procList := []string{}
-	for _, proc := range b.processors.list {
-		procList = append(procList, proc.String())
+
+	if b.processors != nil {
+		for _, proc := range b.processors.list {
+			procList = append(procList, proc.String())
+		}
 	}
+
 	return procList
 }
 
diff --git a/libbeat/publisher/processing/default_test.go b/libbeat/publisher/processing/default_test.go
index ef58dc97b8e..6fe05785037 100644
--- a/libbeat/publisher/processing/default_test.go
+++ b/libbeat/publisher/processing/default_test.go
@@ -480,6 +480,14 @@ func TestProcessingClose(t *testing.T) {
 	assert.True(t, factoryProcessor.closed)
 }
 
+func TestProcessingDiagnostics(t *testing.T) {
+	factory, err := MakeDefaultSupport(true, nil)(beat.Info{}, logp.L(), config.NewConfig())
+	require.NoError(t, err)
+
+	p := factory.Processors()
+	assert.Empty(t, p)
+}
+
 func fromJSON(in string) mapstr.M {
 	var tmp mapstr.M
 	err := json.Unmarshal([]byte(in), &tmp)