diff --git a/drivers/at24cxxx/mtd/mtd.c b/drivers/at24cxxx/mtd/mtd.c
index 47f581d112..73b530a98c 100644
--- a/drivers/at24cxxx/mtd/mtd.c
+++ b/drivers/at24cxxx/mtd/mtd.c
@@ -41,6 +41,7 @@ static int _mtd_at24cxxx_init(mtd_dev_t *mtd)
     mtd->pages_per_sector = 1;
     mtd->sector_count = DEV(mtd)->params.eeprom_size /
                         DEV(mtd)->params.page_size;
+    mtd->write_size = 1;
     return 0;
 }
 
diff --git a/drivers/at25xxx/mtd/mtd.c b/drivers/at25xxx/mtd/mtd.c
index 37d237a3e6..4f96cb3470 100644
--- a/drivers/at25xxx/mtd/mtd.c
+++ b/drivers/at25xxx/mtd/mtd.c
@@ -38,6 +38,7 @@ static int mtd_at25xxx_init(mtd_dev_t *dev)
         dev->pages_per_sector = 1;
         dev->page_size        = mtd_at25xxx->params->page_size;
         dev->sector_count     = mtd_at25xxx->params->size / mtd_at25xxx->params->page_size;
+        dev->write_size       = 1;
         return 0;
     }
     return -EIO;