diff --git a/fints4javaLib/src/main/kotlin/net/dankito/fints/FinTsClient.kt b/fints4javaLib/src/main/kotlin/net/dankito/fints/FinTsClient.kt
index 8c9f4eac..8b7b664c 100644
--- a/fints4javaLib/src/main/kotlin/net/dankito/fints/FinTsClient.kt
+++ b/fints4javaLib/src/main/kotlin/net/dankito/fints/FinTsClient.kt
@@ -2,6 +2,7 @@ package net.dankito.fints
 
 import net.dankito.fints.messages.MessageBuilder
 import net.dankito.fints.messages.datenelemente.implementierte.Dialogsprache
+import net.dankito.fints.messages.datenelemente.implementierte.KundensystemID
 import net.dankito.fints.messages.datenelemente.implementierte.KundensystemStatusWerte
 import net.dankito.fints.model.*
 import net.dankito.fints.response.InstituteSegmentId
@@ -65,30 +66,12 @@ open class FinTsClient(
     }
 
 
-    // TODO: i don't think this method is publicly needed
-    open fun synchronizeCustomerSystemId(bank: BankData, customer: CustomerData): FinTsClientResponse {
-
-        val dialogData = DialogData()
-        val requestBody = messageBuilder.createSynchronizeCustomerSystemIdMessage(bank, customer, product, dialogData)
-
-        val response = getAndHandleResponseForMessage(requestBody, bank)
-
-        if (response.successful) {
-            updateBankData(bank, response)
-            updateCustomerData(customer, response)
-
-            response.messageHeader?.let { header -> dialogData.dialogId = header.dialogId }
-
-            closeDialog(bank, customer, dialogData)
-        }
-
-        return FinTsClientResponse(response)
-    }
-
-
     open fun getTransactions(parameter: GetTransactionsParameter, bank: BankData,
                              customer: CustomerData): GetTransactionsResponse {
 
+//        synchronizeCustomerSystemIdIfNotDoneYet(bank, customer) // even though specification says this is required it can be omitted
+
+
         val dialogData = DialogData()
 
         val initDialogResponse = initDialog(bank, customer, dialogData)
@@ -189,6 +172,47 @@ open class FinTsClient(
     }
 
 
+    protected open fun synchronizeCustomerSystemIdIfNotDoneYet(bank: BankData,
+                                                               customer: CustomerData): FinTsClientResponse {
+
+        if (customer.customerSystemId == KundensystemID.Anonymous) { // customer system id not synchronized yet
+            return synchronizeCustomerSystemId(bank, customer)
+        }
+
+        return FinTsClientResponse(true, false)
+    }
+
+    /**
+     * According to specification synchronizing customer system id is required:
+     * "Die Kundensystem-ID ist beim HBCI RAH- / RDH- sowie dem PIN/TAN-Verfahren erforderlich."
+     *
+     * But as tests show this can be omitted.
+     *
+     * But when you do it, this has to be done in an extra dialog as dialog has to be initialized
+     * with retrieved customer system id.
+     *
+     * If you change customer system id during a dialog your messages get rejected by bank institute.
+     */
+    protected open fun synchronizeCustomerSystemId(bank: BankData, customer: CustomerData): FinTsClientResponse {
+
+        val dialogData = DialogData()
+        val requestBody = messageBuilder.createSynchronizeCustomerSystemIdMessage(bank, customer, product, dialogData)
+
+        val response = getAndHandleResponseForMessage(requestBody, bank)
+
+        if (response.successful) {
+            updateBankData(bank, response)
+            updateCustomerData(customer, response)
+
+            response.messageHeader?.let { header -> dialogData.dialogId = header.dialogId }
+
+            closeDialog(bank, customer, dialogData)
+        }
+
+        return FinTsClientResponse(response)
+    }
+
+
     protected open fun getAndHandleResponseForMessage(requestBody: String, bank: BankData): Response {
         val webResponse = getResponseForMessage(requestBody, bank)
 
diff --git a/fints4javaLib/src/test/kotlin/net/dankito/fints/FinTsClientTest.kt b/fints4javaLib/src/test/kotlin/net/dankito/fints/FinTsClientTest.kt
index 8ad97a5c..b8bb90ea 100644
--- a/fints4javaLib/src/test/kotlin/net/dankito/fints/FinTsClientTest.kt
+++ b/fints4javaLib/src/test/kotlin/net/dankito/fints/FinTsClientTest.kt
@@ -6,6 +6,7 @@ import net.dankito.fints.messages.datenelemente.implementierte.KundensystemStatu
 import net.dankito.fints.messages.datenelemente.implementierte.KundensystemStatusWerte
 import net.dankito.fints.messages.datenelemente.implementierte.signatur.Sicherheitsfunktion
 import net.dankito.fints.model.*
+import net.dankito.fints.response.client.FinTsClientResponse
 import net.dankito.fints.util.Java8Base64Service
 import org.assertj.core.api.Assertions.assertThat
 import org.junit.Ignore
@@ -16,7 +17,13 @@ import java.util.*
 @Ignore // not an automatic test, supply your settings below
 class FinTsClientTest {
 
-    private val underTest = FinTsClient(Java8Base64Service())
+    private val underTest = object : FinTsClient(Java8Base64Service()) {
+
+        fun synchronizeCustomerSystemId(customer: CustomerData, bank: BankData): FinTsClientResponse {
+            return synchronizeCustomerSystemId(bank, customer)
+        }
+
+    }
 
 
     private val BankDataAnonymous = BankData("10070000", Laenderkennzeichen.Germany, "https://fints.deutsche-bank.de/")
@@ -46,7 +53,7 @@ class FinTsClientTest {
     fun synchronizeCustomerSystemId() {
 
         // when
-        val result = underTest.synchronizeCustomerSystemId(Bank, Customer)
+        val result = underTest.synchronizeCustomerSystemId(Customer, Bank)
 
         // then
         assertThat(result.successful).isTrue()