diff --git a/src/class/cdc/cdc_device.c b/src/class/cdc/cdc_device.c
index 681a1b852..792aedf26 100644
--- a/src/class/cdc/cdc_device.c
+++ b/src/class/cdc/cdc_device.c
@@ -81,8 +81,13 @@ typedef struct {
 CFG_TUD_MEM_SECTION static cdcd_interface_t _cdcd_itf[CFG_TUD_CDC];
 static tud_cdc_configure_fifo_t _cdcd_fifo_cfg;
 
-static bool _prep_out_transaction (cdcd_interface_t* p_cdc) {
+static bool _prep_out_transaction (cdcd_interface_t* p_cdc, bool itf_open)
+{
   uint8_t const rhport = 0;
+
+  // Skip if usb is not ready yet
+  TU_VERIFY(tud_ready() || itf_open);
+
   uint16_t available = tu_fifo_remaining(&p_cdc->rx_ff);
 
   // Prepare for incoming data but only allow what we can store in the ring buffer.
@@ -143,7 +148,7 @@ uint32_t tud_cdc_n_available(uint8_t itf) {
 uint32_t tud_cdc_n_read(uint8_t itf, void* buffer, uint32_t bufsize) {
   cdcd_interface_t* p_cdc = &_cdcd_itf[itf];
   uint32_t num_read = tu_fifo_read_n(&p_cdc->rx_ff, buffer, (uint16_t) TU_MIN(bufsize, UINT16_MAX));
-  _prep_out_transaction(p_cdc);
+  _prep_out_transaction(p_cdc, false);
   return num_read;
 }
 
@@ -154,7 +159,7 @@ bool tud_cdc_n_peek(uint8_t itf, uint8_t* chr) {
 void tud_cdc_n_read_flush(uint8_t itf) {
   cdcd_interface_t* p_cdc = &_cdcd_itf[itf];
   tu_fifo_clear(&p_cdc->rx_ff);
-  _prep_out_transaction(p_cdc);
+  _prep_out_transaction(p_cdc, false);
 }
 
 //--------------------------------------------------------------------+
@@ -336,7 +341,7 @@ uint16_t cdcd_open(uint8_t rhport, tusb_desc_interface_t const * itf_desc, uint1
   }
 
   // Prepare for incoming data
-  _prep_out_transaction(p_cdc);
+  _prep_out_transaction(p_cdc, true);
 
   return drv_len;
 }
@@ -445,7 +450,7 @@ bool cdcd_xfer_cb(uint8_t rhport, uint8_t ep_addr, xfer_result_t result, uint32_
     if (tud_cdc_rx_cb && !tu_fifo_empty(&p_cdc->rx_ff)) tud_cdc_rx_cb(itf);
 
     // prepare for OUT transaction
-    _prep_out_transaction(p_cdc);
+    _prep_out_transaction(p_cdc, false);
   }
 
   // Data sent to host, we continue to fetch from tx fifo to send.