diff --git a/voicemessage/voice_message.go b/voicemessage/voice_message.go
index 3b8a6c8..5d47f43 100644
--- a/voicemessage/voice_message.go
+++ b/voicemessage/voice_message.go
@@ -119,7 +119,9 @@ func paramsToRequest(recipients []string, body string, params *Params) (*voiceMe
 	request.Voice = params.Voice
 	request.Repeat = params.Repeat
 	request.IfMachine = params.IfMachine
-	request.ScheduledDatetime = params.ScheduledDatetime.Format(time.RFC3339)
+	if !params.ScheduledDatetime.IsZero() {
+		request.ScheduledDatetime = params.ScheduledDatetime.Format(time.RFC3339)
+	}
 
 	return request, nil
 }
diff --git a/voicemessage/voice_message_test.go b/voicemessage/voice_message_test.go
index fbbd440..2ddfcfc 100644
--- a/voicemessage/voice_message_test.go
+++ b/voicemessage/voice_message_test.go
@@ -5,7 +5,7 @@ import (
 	"testing"
 	"time"
 
-	"github.com/messagebird/go-rest-api/v9"
+	messagebird "github.com/messagebird/go-rest-api/v9"
 	"github.com/messagebird/go-rest-api/v9/internal/mbtest"
 	"github.com/stretchr/testify/assert"
 )
@@ -124,3 +124,13 @@ func TestRequestDataForVoiceMessage(t *testing.T) {
 	assert.Equal(t, "continue", request.IfMachine)
 	assert.Equal(t, voiceParams.ScheduledDatetime.Format(time.RFC3339), request.ScheduledDatetime)
 }
+
+func TestRequestDataForVoiceMessageZeroScheduledDatetime(t *testing.T) {
+	voiceParams := &Params{
+		// ScheduledDatetime will default to zero-time
+	}
+
+	request, err := paramsToRequest([]string{"31612345678"}, "MyBody", voiceParams)
+	assert.NoError(t, err)
+	assert.Equal(t, "", request.ScheduledDatetime, "Uninitialized ScheduledDatetime should default to empty string")
+}