diff --git a/kern/proc.h b/kern/proc.h
index 489876e..ca2fa93 100644
--- a/kern/proc.h
+++ b/kern/proc.h
@@ -5,7 +5,7 @@
 #include <riscv.h>
 #include <stdint.h>
 
-typedef enum {
+typedef enum : uint8_t {
     UNUSED,
     USED,
     SLEEPING,
@@ -14,6 +14,8 @@ typedef enum {
     ZOMBIE,
 } ProcessState;
 
+_Static_assert(sizeof(ProcessState) == 1, "");
+
 /** Saved registers for kernel context switches. */
 typedef struct Context {
     uint64_t ra;