diff --git a/UPGRADING b/UPGRADING
index 596db8fb4267..41d9fd6b97a5 100644
--- a/UPGRADING
+++ b/UPGRADING
@@ -48,6 +48,12 @@ PHP 8.6 UPGRADE NOTES
     the integer index is greater than INT_MAX instead of overflowing to a
     smaller index.
 
+- FTP:
+  . ftp_nb_fget() and ftp_nb_fput() now throw an Error when the connection is
+    already transferring, instead of emitting a warning and returning false
+    against their declared int return type. ftp_close() already throws on the
+    same condition.
+
 - GD:
   . imagesetstyle(), imagefilter() and imagecrop() filter the types / values of
     their array arguments and raise a TypeError / ValueError accordingly.
diff --git a/ext/ftp/php_ftp.c b/ext/ftp/php_ftp.c
index 56938459bb33..36f60f93a049 100644
--- a/ext/ftp/php_ftp.c
+++ b/ext/ftp/php_ftp.c
@@ -656,8 +656,8 @@ PHP_FUNCTION(ftp_nb_fget)
 
 	/* configuration */
 	if (ftp->in_use) {
-		php_error_docref(NULL, E_WARNING, "FTP\\Connection is already in use");
-		RETURN_FALSE;
+		zend_throw_error(NULL, "Cannot start a transfer while another transfer is in progress");
+		RETURN_THROWS();
 	}
 
 	ftp->direction = 0;   /* recv */
@@ -961,8 +961,8 @@ PHP_FUNCTION(ftp_nb_fput)
 
 	/* configuration */
 	if (ftp->in_use) {
-		php_error_docref(NULL, E_WARNING, "FTP\\Connection is already in use");
-		RETURN_FALSE;
+		zend_throw_error(NULL, "Cannot start a transfer while another transfer is in progress");
+		RETURN_THROWS();
 	}
 
 	ftp->direction = true;   /* send */
diff --git a/ext/ftp/tests/ftp_nb_transfer_during_transfer.phpt b/ext/ftp/tests/ftp_nb_transfer_during_transfer.phpt
new file mode 100644
index 000000000000..c15b713d6e4a
--- /dev/null
+++ b/ext/ftp/tests/ftp_nb_transfer_during_transfer.phpt
@@ -0,0 +1,56 @@
+--TEST--
+ftp_nb_fget() and ftp_nb_fput() throw when a transfer is already in progress
+--EXTENSIONS--
+ftp
+pcntl
+--FILE--
+<?php
+require 'server.inc';
+
+class TransferDuringNbWrite {
+    public $context;
+    public static $ftp;
+    public static $call;
+    public function stream_open($path, $mode, $options, &$opened_path) {
+        return true;
+    }
+    public function stream_write($data) {
+        try {
+            (self::$call)(self::$ftp);
+        } catch (\Error $e) {
+            echo $e::class, ': ', $e->getMessage(), "\n";
+        }
+        return strlen($data);
+    }
+    public function stream_close() {}
+    public function stream_eof() {
+        return true;
+    }
+}
+
+stream_wrapper_register('reentrantnb', TransferDuringNbWrite::class);
+
+$ftp = ftp_connect('127.0.0.1', $port);
+var_dump(ftp_login($ftp, 'user', 'pass'));
+TransferDuringNbWrite::$ftp = $ftp;
+
+$sink = fopen('php://memory', 'w+');
+
+TransferDuringNbWrite::$call = static function ($ftp) use ($sink) {
+    ftp_nb_fget($ftp, $sink, 'a story.txt', FTP_BINARY);
+};
+@ftp_nb_get($ftp, 'reentrantnb://sink', 'a story.txt', FTP_BINARY);
+
+TransferDuringNbWrite::$call = static function ($ftp) use ($sink) {
+    ftp_nb_fput($ftp, 'a story.txt', $sink, FTP_BINARY);
+};
+@ftp_nb_get($ftp, 'reentrantnb://sink', 'a story.txt', FTP_BINARY);
+
+ftp_close($ftp);
+echo "closed\n";
+?>
+--EXPECT--
+bool(true)
+Error: Cannot start a transfer while another transfer is in progress
+Error: Cannot start a transfer while another transfer is in progress
+closed