diff --git a/os/Makefile b/os/Makefile
index 8aa64b9e..2bc72342 100644
--- a/os/Makefile
+++ b/os/Makefile
@@ -1,6 +1,6 @@
 # Building
 TARGET := riscv64gc-unknown-none-elf
-MODE ?= release
+MODE := release
 KERNEL_ELF := target/$(TARGET)/$(MODE)/os
 KERNEL_BIN := $(KERNEL_ELF).bin
 DISASM_TMP := target/$(TARGET)/$(MODE)/asm