diff --git a/esp8266/modpybrtc.c b/esp8266/modpybrtc.c
index 6c2dde691e82efdba79b3d8ec2163038ffa6528d..6dea4b1b90f1bddc960010749c81221102e487ad 100644
--- a/esp8266/modpybrtc.c
+++ b/esp8266/modpybrtc.c
@@ -146,7 +146,7 @@ STATIC mp_obj_t pyb_rtc_memory(mp_uint_t n_args, const mp_obj_t *args) {
         }
 
         magic = MEM_MAGIC;
-        system_rtc_mem_write(MEM_USER_MAGIC_ADDR, &magic, sizeof(len));
+        system_rtc_mem_write(MEM_USER_MAGIC_ADDR, &magic, sizeof(magic));
         len = bufinfo.len;
         system_rtc_mem_write(MEM_USER_LEN_ADDR, &len, sizeof(len));