diff --git a/esp8266/modpybrtc.c b/esp8266/modpybrtc.c
index 6c2dde6..6dea4b1 100644
--- a/esp8266/modpybrtc.c
+++ b/esp8266/modpybrtc.c
@@ -146,7 +146,7 @@
         }
 
         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));