diff --git a/targets/stm32l442/src/flash.c b/targets/stm32l442/src/flash.c
index fafe136..f6254a8 100644
--- a/targets/stm32l442/src/flash.c
+++ b/targets/stm32l442/src/flash.c
@@ -141,5 +141,5 @@ void flash_write(uint32_t addr, uint8_t * data, size_t sz)
 
 void flash_lock()
 {
-    FLASH->CR |= (1<<31);
+    FLASH->CR |= (1U<<31);
 }