diff --git a/lib/stm32/f1/flash.c b/lib/stm32/f1/flash.c
index 49f182c6..babfc45b 100644
--- a/lib/stm32/f1/flash.c
+++ b/lib/stm32/f1/flash.c
@@ -52,6 +52,9 @@ void flash_set_ws(u32 ws)
 
 void flash_unlock(void)
 {
+	/* Clear the unlock state. */
+	FLASH_CR |= FLASH_CR_LOCK;
+
 	/* Authorize the FPEC access. */
 	FLASH_KEYR = FLASH_KEYR_KEY1;
 	FLASH_KEYR = FLASH_KEYR_KEY2;
diff --git a/lib/stm32/f2/flash.c b/lib/stm32/f2/flash.c
index 0b9f8348..c41ddf2c 100644
--- a/lib/stm32/f2/flash.c
+++ b/lib/stm32/f2/flash.c
@@ -78,6 +78,9 @@ void flash_set_ws(u32 ws)
 
 void flash_unlock(void)
 {
+	/* Clear the unlock state. */
+	FLASH_CR |= FLASH_CR_LOCK;
+
 	/* Authorize the FPEC access. */
 	FLASH_KEYR = FLASH_KEYR_KEY1;
 	FLASH_KEYR = FLASH_KEYR_KEY2;
@@ -130,6 +133,10 @@ void flash_clear_status_flags(void)
 
 void flash_unlock_option_bytes(void)
 {
+	/* Clear the unlock state. */
+	FLASH_OPTCR |= FLASH_OPTCR_OPTLOCK;
+
+	/* Unlock the option bytes. */
 	FLASH_OPTKEYR = FLASH_OPTKEYR_KEY1;
 	FLASH_OPTKEYR = FLASH_OPTKEYR_KEY2;
 }
diff --git a/lib/stm32/f4/flash.c b/lib/stm32/f4/flash.c
index 75446ea2..a2045436 100644
--- a/lib/stm32/f4/flash.c
+++ b/lib/stm32/f4/flash.c
@@ -78,6 +78,9 @@ void flash_set_ws(u32 ws)
 
 void flash_unlock(void)
 {
+	/* Clear the unlock sequence state. */
+	FLASH_CR |= FLASH_CR_LOCK;
+
 	/* Authorize the FPEC access. */
 	FLASH_KEYR = FLASH_KEYR_KEY1;
 	FLASH_KEYR = FLASH_KEYR_KEY2;
@@ -130,6 +133,10 @@ void flash_clear_status_flags(void)
 
 void flash_unlock_option_bytes(void)
 {
+	/* Clear the unlock state. */
+	FLASH_OPTCR |= FLASH_OPTCR_OPTLOCK;
+
+	/* Unlock option bytes. */
 	FLASH_OPTKEYR = FLASH_OPTKEYR_KEY1;
 	FLASH_OPTKEYR = FLASH_OPTKEYR_KEY2;
 }