diff --git a/ext/ftp/ftp.c b/ext/ftp/ftp.c
index ef91fdec97988..09367097cdb5d 100644
--- a/ext/ftp/ftp.c
+++ b/ext/ftp/ftp.c
@@ -867,7 +867,7 @@ ftp_get(ftpbuf_t *ftp, php_stream *outstream, const char *path, const size_t pat
 {
 	databuf_t		*data = NULL;
 	size_t			rcvd;
-	char			arg[11];
+	char			arg[MAX_LENGTH_OF_LONG];
 
 	if (ftp == NULL) {
 		return 0;
@@ -964,7 +964,7 @@ ftp_put(ftpbuf_t *ftp, const char *path, const size_t path_len, php_stream *inst
 	zend_long			size;
 	char			*ptr;
 	int			ch;
-	char			arg[11];
+	char			arg[MAX_LENGTH_OF_LONG];
 
 	if (ftp == NULL) {
 		return 0;
@@ -2057,7 +2057,7 @@ int
 ftp_nb_get(ftpbuf_t *ftp, php_stream *outstream, const char *path, const size_t path_len, ftptype_t type, zend_long resumepos)
 {
 	databuf_t		*data = NULL;
-	char			arg[11];
+	char			arg[MAX_LENGTH_OF_LONG];
 
 	if (ftp == NULL) {
 		return PHP_FTP_FAILED;
@@ -2176,7 +2176,7 @@ int
 ftp_nb_put(ftpbuf_t *ftp, const char *path, const size_t path_len, php_stream *instream, ftptype_t type, zend_long startpos)
 {
 	databuf_t		*data = NULL;
-	char			arg[11];
+	char			arg[MAX_LENGTH_OF_LONG];
 
 	if (ftp == NULL) {
 		return 0;
diff --git a/ext/ftp/tests/gh10521.phpt b/ext/ftp/tests/gh10521.phpt
new file mode 100644
index 0000000000000..1dc40d4004271
--- /dev/null
+++ b/ext/ftp/tests/gh10521.phpt
@@ -0,0 +1,39 @@
+--TEST--
+GH-10521 (ftp_get/ftp_nb_get resumepos offset is maximum 10GB)
+--EXTENSIONS--
+ftp
+pcntl
+--SKIPIF--
+<?php
+if (PHP_INT_SIZE != 8) die("skip: 64-bit only");
+?>
+--FILE--
+<?php
+require 'server.inc';
+
+$ftp = ftp_connect('127.0.0.1', $port);
+if (!$ftp) die("Couldn't connect to the server");
+
+var_dump(ftp_login($ftp, 'anonymous', 'IEUser@'));
+
+$local_file = __DIR__ . DIRECTORY_SEPARATOR . "gh10521.txt";
+
+foreach ([12345678910, 9223372036854775807] as $size) {
+    $handle = fopen($local_file, 'w');
+    // Doesn't actually succeed in transferring a file. The file transfer gets aborted by our fake server.
+    // We just want to see if the offset was correctly received.
+    ftp_fget($ftp, $handle, 'gh10521', FTP_ASCII, $size);
+    fclose($handle);
+}
+
+?>
+--CLEAN--
+<?php
+@unlink(__DIR__ . DIRECTORY_SEPARATOR . "gh10521.txt");
+?>
+--EXPECTF--
+bool(true)
+
+%s: ftp_fget(): Can't open data connection (12345678910). in %s on line %d
+
+%s: ftp_fget(): Can't open data connection (9223372036854775807). in %s on line %d
diff --git a/ext/ftp/tests/server.inc b/ext/ftp/tests/server.inc
index afbfd88f73579..bdae0278793f7 100644
--- a/ext/ftp/tests/server.inc
+++ b/ext/ftp/tests/server.inc
@@ -386,6 +386,10 @@ if ($pid) {
                 case "/bug73457":
                     fputs($s, "150 File status okay; about to open data connection.\r\n");
                     break;
+                case "gh10521":
+                    // Just a side channel for getting the received file size.
+                    fputs($s, "425 Can't open data connection (".$GLOBALS['rest_pos'].").\r\n");    
+                    break;
 
                 default:
                     fputs($s, "550 {$matches[1]}: No such file or directory \r\n");