diff --git a/fints4k/src/commonMain/kotlin/net/codinux/banking/fints/tan/FlickerCode.kt b/fints4k/src/commonMain/kotlin/net/codinux/banking/fints/tan/FlickerCode.kt
index 0d43519f..2d27fbfc 100644
--- a/fints4k/src/commonMain/kotlin/net/codinux/banking/fints/tan/FlickerCode.kt
+++ b/fints4k/src/commonMain/kotlin/net/codinux/banking/fints/tan/FlickerCode.kt
@@ -3,12 +3,12 @@ package net.codinux.banking.fints.tan
 
 open class FlickerCode(
     val challengeHHD_UC: String,
-    val parsedDataSet: String,
+    val parsedDataSet: String? = null,
     val decodingError: Exception? = null
 ) {
 
     val decodingSuccessful: Boolean
-        get() = decodingError == null
+        get() = parsedDataSet != null
 
 
     override fun toString(): String {
diff --git a/fints4k/src/commonMain/kotlin/net/codinux/banking/fints/tan/FlickerCodeDecoder.kt b/fints4k/src/commonMain/kotlin/net/codinux/banking/fints/tan/FlickerCodeDecoder.kt
index b5db757f..2237fecb 100644
--- a/fints4k/src/commonMain/kotlin/net/codinux/banking/fints/tan/FlickerCodeDecoder.kt
+++ b/fints4k/src/commonMain/kotlin/net/codinux/banking/fints/tan/FlickerCodeDecoder.kt
@@ -45,7 +45,7 @@ open class FlickerCodeDecoder {
         } catch (e: Exception) {
             log.error(e) { "Could not decode challenge $challengeHHD_UC" }
 
-            return FlickerCode(challengeHHD_UC, "", e)
+            return FlickerCode(challengeHHD_UC, null, e)
         }
     }
 
diff --git a/fints4k/src/commonMain/kotlin/net/codinux/banking/fints/tan/TanImage.kt b/fints4k/src/commonMain/kotlin/net/codinux/banking/fints/tan/TanImage.kt
index 819ee46d..322a7e26 100644
--- a/fints4k/src/commonMain/kotlin/net/codinux/banking/fints/tan/TanImage.kt
+++ b/fints4k/src/commonMain/kotlin/net/codinux/banking/fints/tan/TanImage.kt
@@ -2,13 +2,13 @@ package net.codinux.banking.fints.tan
 
 
 open class TanImage(
-    val mimeType: String,
-    val imageBytes: ByteArray,
+    val mimeType: String? = null,
+    val imageBytes: ByteArray? = null,
     val decodingError: Exception? = null
 ) {
 
     val decodingSuccessful: Boolean
-        get() = decodingError == null
+        get() = mimeType != null && imageBytes != null
 
 
     override fun toString(): String {
@@ -16,7 +16,7 @@ open class TanImage(
             return "Decoding error: $decodingError"
         }
 
-        return "$mimeType ${imageBytes.size} bytes"
+        return "$mimeType ${imageBytes?.size} bytes"
     }
 
 }
\ No newline at end of file
diff --git a/fints4k/src/commonMain/kotlin/net/codinux/banking/fints/tan/TanImageDecoder.kt b/fints4k/src/commonMain/kotlin/net/codinux/banking/fints/tan/TanImageDecoder.kt
index 8d3b533c..0fe3311c 100644
--- a/fints4k/src/commonMain/kotlin/net/codinux/banking/fints/tan/TanImageDecoder.kt
+++ b/fints4k/src/commonMain/kotlin/net/codinux/banking/fints/tan/TanImageDecoder.kt
@@ -29,7 +29,7 @@ open class TanImageDecoder {
         } catch (e: Exception) {
             log.error(e) { "Could not decode challenge HHD_UC to TanImage: $challengeHHD_UC" }
 
-            return TanImage("", ByteArray(0), e)
+            return TanImage(null, null, e)
         }
     }