diff --git a/libraries/SD/src/sd_diskio.cpp b/libraries/SD/src/sd_diskio.cpp
index 0e3f4c581b5..4f27e628438 100644
--- a/libraries/SD/src/sd_diskio.cpp
+++ b/libraries/SD/src/sd_diskio.cpp
@@ -809,7 +809,7 @@ bool sdcard_mount(uint8_t pdrv, const char* path, uint8_t max_files, bool format
               log_e("alloc for f_mkfs failed");
               return false;
             }
-            res = f_mkfs(drv, FM_ANY, 0, work, sizeof(work));
+            res = f_mkfs(drv, FM_ANY, 0, work, sizeof(BYTE) * FF_MAX_SS);
             free(work);
             if (res != FR_OK) {
                 log_e("f_mkfs failed: %s", fferr2str[res]);