diff --git a/fints4javaLib/src/main/kotlin/net/dankito/fints/response/segments/TanResponse.kt b/fints4javaLib/src/main/kotlin/net/dankito/fints/response/segments/TanResponse.kt
index e72c1d58..6159657c 100644
--- a/fints4javaLib/src/main/kotlin/net/dankito/fints/response/segments/TanResponse.kt
+++ b/fints4javaLib/src/main/kotlin/net/dankito/fints/response/segments/TanResponse.kt
@@ -44,6 +44,6 @@ open class TanResponse(
     }
 
     open val isStrongAuthenticationRequired: Boolean
-        get() = challenge != NoChallengeResponse
+        get() = challenge != null && challenge != NoChallengeResponse
 
 }
\ No newline at end of file