diff --git a/Marlin/src/feature/binary_stream.h b/Marlin/src/feature/binary_stream.h
index d092b7152..80f26cc7c 100644
--- a/Marlin/src/feature/binary_stream.h
+++ b/Marlin/src/feature/binary_stream.h
@@ -29,11 +29,11 @@
   #include "../libs/heatshrink/heatshrink_decoder.h"
 #endif
 
-inline bool bs_serial_data_available(const uint8_t index) {
+inline bool bs_serial_data_available(const serial_index_t index) {
   return SERIAL_IMPL.available(index);
 }
 
-inline int bs_read_serial(const uint8_t index) {
+inline int bs_read_serial(const serial_index_t index) {
   return SERIAL_IMPL.read(index);
 }
 
diff --git a/Marlin/src/gcode/queue.cpp b/Marlin/src/gcode/queue.cpp
index 0f4169a2c..4f676eaeb 100644
--- a/Marlin/src/gcode/queue.cpp
+++ b/Marlin/src/gcode/queue.cpp
@@ -393,7 +393,7 @@ void GCodeQueue::get_serial_commands() {
        * receive buffer (which limits the packet size to MAX_CMD_SIZE).
        * The receive buffer also limits the packet size for reliable transmission.
        */
-      binaryStream[card.transfer_port_index].receive(serial_state[card.transfer_port_index].line_buffer);
+      binaryStream[card.transfer_port_index.index].receive(serial_state[card.transfer_port_index.index].line_buffer);
       return;
     }
   #endif
diff --git a/Marlin/src/sd/cardreader.cpp b/Marlin/src/sd/cardreader.cpp
index fac14e72a..a3e057703 100644
--- a/Marlin/src/sd/cardreader.cpp
+++ b/Marlin/src/sd/cardreader.cpp
@@ -70,7 +70,7 @@ char CardReader::filename[FILENAME_LENGTH], CardReader::longFilename[LONG_FILENA
 IF_DISABLED(NO_SD_AUTOSTART, uint8_t CardReader::autofile_index); // = 0
 
 #if BOTH(HAS_MULTI_SERIAL, BINARY_FILE_TRANSFER)
-  int8_t CardReader::transfer_port_index;
+  serial_index_t CardReader::transfer_port_index;
 #endif
 
 // private:
diff --git a/Marlin/src/sd/cardreader.h b/Marlin/src/sd/cardreader.h
index 0a89bbba7..f823b79e7 100644
--- a/Marlin/src/sd/cardreader.h
+++ b/Marlin/src/sd/cardreader.h
@@ -72,9 +72,9 @@ public:
   // Fast! binary file transfer
   #if ENABLED(BINARY_FILE_TRANSFER)
     #if HAS_MULTI_SERIAL
-      static int8_t transfer_port_index;
+      static serial_index_t transfer_port_index;
     #else
-      static constexpr int8_t transfer_port_index = 0;
+      static constexpr serial_index_t transfer_port_index = 0;
     #endif
   #endif