diff --git a/drivers/flash.c b/drivers/flash.c
index a49df48..3219253 100644
--- a/drivers/flash.c
+++ b/drivers/flash.c
@@ -28,7 +28,7 @@ static volatile struct FLASH* regs = (struct FLASH*)FLASH_BASE_ADDRESS;
 void flash_configure(enum FlashPreset preset)
 {
 	//restore default configuration
-	regs->ACR.word &= !0x3f;
+	regs->ACR.word &= ~0x3f;
 	regs->ACR.word |= 0x30;
 
 	//apply new configuration
diff --git a/drivers/rcc.c b/drivers/rcc.c
index 65c7fcc..c4c7bb0 100644
--- a/drivers/rcc.c
+++ b/drivers/rcc.c
@@ -94,7 +94,7 @@ static void apply_default_preset(void)
 	while (regs->CR.HSIRDY != 0x1);
 
 	//set HSI as main clock source and disable prescalers
-	regs->CFGR.word &= !0x077fff3;
+	regs->CFGR.word &= ~0x077fff3;
 
 	//disable all options
 	regs->CR.HSITRIM = 0x10;