diff --git a/src/com/android/services/telephony/TelephonyConnection.java b/src/com/android/services/telephony/TelephonyConnection.java
index 0fe6980..af9e9d5 100644
--- a/src/com/android/services/telephony/TelephonyConnection.java
+++ b/src/com/android/services/telephony/TelephonyConnection.java
@@ -1698,6 +1698,7 @@
     private boolean isRtt() {
         return mOriginalConnection != null
                 && mOriginalConnection.getPhoneType() == PhoneConstants.PHONE_TYPE_IMS
+                && mOriginalConnection instanceof ImsPhoneConnection
                 && ((ImsPhoneConnection) mOriginalConnection).isRttEnabledForCall();
     }