diff --git a/method.h b/method.h
index 0cb1b4135a..3e424fbc02 100644
--- a/method.h
+++ b/method.h
@@ -114,6 +114,8 @@ typedef enum {
 
     END_OF_ENUMERATION(VM_METHOD_TYPE)
 } rb_method_type_t;
+#define VM_METHOD_TYPE_MINIMUM_BITS 4
+/* TODO: STATIC_ASSERT for VM_METHOD_TYPE_MINIMUM_BITS */
 
 #ifndef rb_iseq_t
 typedef struct rb_iseq_struct rb_iseq_t;
@@ -153,7 +155,7 @@ enum method_optimized_type {
 };
 
 PACKED_STRUCT_UNALIGNED(struct rb_method_definition_struct {
-    BITFIELD(rb_method_type_t) type : 4;
+    BITFIELD(rb_method_type_t) type : VM_METHOD_TYPE_MINIMUM_BITS;
     int alias_count : 28;
     int complemented_count : 28;