diff --git a/fints4javaAndroidApp/src/main/java/net/dankito/banking/fints4java/android/ui/dialogs/EnterTanDialog.kt b/fints4javaAndroidApp/src/main/java/net/dankito/banking/fints4java/android/ui/dialogs/EnterTanDialog.kt
index 54c15bd1..ec32394b 100644
--- a/fints4javaAndroidApp/src/main/java/net/dankito/banking/fints4java/android/ui/dialogs/EnterTanDialog.kt
+++ b/fints4javaAndroidApp/src/main/java/net/dankito/banking/fints4java/android/ui/dialogs/EnterTanDialog.kt
@@ -3,6 +3,7 @@ package net.dankito.banking.fints4java.android.ui.dialogs
 import android.content.Context
 import android.graphics.BitmapFactory
 import android.os.Bundle
+import android.os.Handler
 import android.support.v4.app.DialogFragment
 import android.support.v7.app.AlertDialog
 import android.support.v7.app.AppCompatActivity
@@ -28,6 +29,7 @@ import net.dankito.fints.model.TanChallenge
 import net.dankito.fints.model.TanProcedureType
 import net.dankito.fints.response.client.FinTsClientResponse
 import net.dankito.fints.tan.FlickercodeDecoder
+import net.dankito.fints.tan.TanImage
 import net.dankito.fints.tan.TanImageDecoder
 
 
@@ -144,11 +146,14 @@ open class EnterTanDialog : DialogFragment() {
                 || tanChallenge.tanProcedure.type == TanProcedureType.PhotoTan) {
                 rootView.tanImageView.visibility = View.VISIBLE
 
-                TanImageDecoder().decodeChallenge(tanChallenge.tanChallenge)?.let { tanImage ->
-                    val bitmap = BitmapFactory.decodeByteArray(tanImage.imageBytes, 0, tanImage.imageBytes.size)
+                val decodedImage = TanImageDecoder().decodeChallenge(tanChallenge.tanChallenge)
+                if (decodedImage.decodingSuccessful) {
+                    val bitmap = BitmapFactory.decodeByteArray(decodedImage.imageBytes, 0, decodedImage.imageBytes.size)
                     rootView.imgTanImageView.setImageBitmap(bitmap)
                 }
-                // TODO: what to do if decoding fails? At least a message should get shown to user
+                else {
+                    showDecodingTanImageFailedErrorDelayed(decodedImage) // this method gets called right on start up before dialog is shown -> Alert would get displayed before dialog and therefore covered by dialog if we don't delay displaying dialog
+                }
             }
 
             rootView.edtxtEnteredTan.inputType = InputType.TYPE_CLASS_NUMBER
@@ -156,6 +161,22 @@ open class EnterTanDialog : DialogFragment() {
     }
 
 
+    protected open fun showDecodingTanImageFailedErrorDelayed(decodedImage: TanImage) {
+        val handler = Handler()
+
+        handler.postDelayed({ showDecodingTanImageFailedError(decodedImage) }, 500)
+    }
+
+    protected open fun showDecodingTanImageFailedError(decodedImage: TanImage) {
+        activity?.let { context ->
+            AlertDialog.Builder(context)
+                .setMessage(context.getString(R.string.dialog_enter_tan_error_could_not_decode_tan_image, decodedImage.error?.localizedMessage))
+                .setPositiveButton(android.R.string.ok) { dialog, _ -> dialog.dismiss() }
+                .show()
+        }
+    }
+
+
     protected open fun handleChangeTanMediumResponse(newUsedTanMedium: TanMedium, response: FinTsClientResponse) {
         activity?.let { activity ->
             activity.runOnUiThread {
diff --git a/fints4javaAndroidApp/src/main/res/values/strings.xml b/fints4javaAndroidApp/src/main/res/values/strings.xml
index 798bd4de..09c93272 100644
--- a/fints4javaAndroidApp/src/main/res/values/strings.xml
+++ b/fints4javaAndroidApp/src/main/res/values/strings.xml
@@ -61,6 +61,7 @@
     <string name="dialog_enter_tan_select_tan_procedure">TAN procedure</string>
     <string name="dialog_enter_tan_select_tan_medium">TAN medium</string>
     <string name="dialog_enter_tan_enter_tan">Enter TAN:</string>
+    <string name="dialog_enter_tan_error_could_not_decode_tan_image">Could not decode QR code / PhotoTan. Most likely an internal error:\n%s.</string>
     <string name="dialog_enter_tan_tan_medium_successfully_changed">TAN medium successfully changed to \'%s\'.</string>
     <string name="dialog_enter_tan_error_changing_tan_medium">Could not change TAN medium to \'%s\':\n%s.</string>
 
diff --git a/fints4javaLib/src/main/kotlin/net/dankito/fints/tan/TanImage.kt b/fints4javaLib/src/main/kotlin/net/dankito/fints/tan/TanImage.kt
index 66f31424..83a5f827 100644
--- a/fints4javaLib/src/main/kotlin/net/dankito/fints/tan/TanImage.kt
+++ b/fints4javaLib/src/main/kotlin/net/dankito/fints/tan/TanImage.kt
@@ -3,9 +3,14 @@ package net.dankito.fints.tan
 
 open class TanImage(
     val mimeType: String,
-    val imageBytes: ByteArray
+    val imageBytes: ByteArray,
+    val error: Exception? = null
 ) {
 
+    val decodingSuccessful: Boolean
+        get() = error == null
+
+
     override fun toString(): String {
         return "$mimeType ${imageBytes.size} bytes"
     }
diff --git a/fints4javaLib/src/main/kotlin/net/dankito/fints/tan/TanImageDecoder.kt b/fints4javaLib/src/main/kotlin/net/dankito/fints/tan/TanImageDecoder.kt
index b0a039c8..95ed120c 100644
--- a/fints4javaLib/src/main/kotlin/net/dankito/fints/tan/TanImageDecoder.kt
+++ b/fints4javaLib/src/main/kotlin/net/dankito/fints/tan/TanImageDecoder.kt
@@ -11,7 +11,7 @@ open class TanImageDecoder {
     }
 
 
-    open fun decodeChallenge(challengeHHD_UC: String): TanImage? {
+    open fun decodeChallenge(challengeHHD_UC: String): TanImage {
         try {
             val bytes = challengeHHD_UC.toByteArray(HbciCharset.DefaultCharset)
 
@@ -29,9 +29,9 @@ open class TanImageDecoder {
             return TanImage(mimeType, imageBytes)
         } catch (e: Exception) {
             log.error("Could not decode challenge HHD_UC to TanImage: $challengeHHD_UC", e)
-        }
 
-        return null
+            return TanImage("", ByteArray(0), e)
+        }
     }
 
     protected open fun getLength(higherOrderByte: Byte, lowerOrderByte: Byte): Int {