diff --git a/src/class/cdc/cdc.h b/src/class/cdc/cdc.h
index 0b6e57735..afac8379d 100644
--- a/src/class/cdc/cdc.h
+++ b/src/class/cdc/cdc.h
@@ -90,7 +90,8 @@ typedef enum
 /// Communication Interface Protocol Codes
 typedef enum
 {
-  CDC_COMM_PROTOCOL_ATCOMMAND              = 0x01 , ///< AT Commands: V.250 etc
+  CDC_COMM_PROTOCOL_NONE                   = 0x00 , ///< No specific protocol
+  CDC_COMM_PROTOCOL_ATCOMMAND                     , ///< AT Commands: V.250 etc
   CDC_COMM_PROTOCOL_ATCOMMAND_PCCA_101            , ///< AT Commands defined by PCCA-101
   CDC_COMM_PROTOCOL_ATCOMMAND_PCCA_101_AND_ANNEXO , ///< AT Commands defined by PCCA-101 & Annex O
   CDC_COMM_PROTOCOL_ATCOMMAND_GSM_707             , ///< AT Commands defined by GSM 07.07
diff --git a/src/class/cdc/cdc_device.c b/src/class/cdc/cdc_device.c
index ab9addc93..49e13c4b7 100644
--- a/src/class/cdc/cdc_device.c
+++ b/src/class/cdc/cdc_device.c
@@ -227,8 +227,10 @@ tusb_error_t cdcd_open(uint8_t rhport, tusb_desc_interface_t const * p_interface
 {
   if ( CDC_COMM_SUBCLASS_ABSTRACT_CONTROL_MODEL != p_interface_desc->bInterfaceSubClass) return TUSB_ERROR_CDC_UNSUPPORTED_SUBCLASS;
 
+  // Only support AT commands, no protocol and vendor specific commands.
   if ( !(tu_within(CDC_COMM_PROTOCOL_ATCOMMAND, p_interface_desc->bInterfaceProtocol, CDC_COMM_PROTOCOL_ATCOMMAND_CDMA) ||
-         0xff == p_interface_desc->bInterfaceProtocol) )
+         p_interface_desc->bInterfaceProtocol == CDC_COMM_PROTOCOL_NONE ||
+         p_interface_desc->bInterfaceProtocol == 0xff ) )
   {
     return TUSB_ERROR_CDC_UNSUPPORTED_PROTOCOL;
   }