diff --git a/include/ruby/internal/special_consts.h b/include/ruby/internal/special_consts.h
index 38934e4da3..70276278eb 100644
--- a/include/ruby/internal/special_consts.h
+++ b/include/ruby/internal/special_consts.h
@@ -136,6 +136,7 @@ static inline bool
 RB_TEST(VALUE obj)
 {
     /*
+     * if USE_FLONUM
      *  Qfalse:  ....0000 0000
      *  Qnil:    ....0000 1000
      * ~Qnil:    ....1111 0111
@@ -143,6 +144,14 @@ RB_TEST(VALUE obj)
      * ----------------------------
      *  RTEST(v) ....xxxx 0xxx
      *
+     * if ! USE_FLONUM
+     *  Qfalse:  ....0000 0000
+     *  Qnil:    ....0000 0100
+     * ~Qnil:    ....1111 1011
+     *  v        ....xxxx xxxx
+     * ----------------------------
+     *  RTEST(v) ....xxxx x0xx
+     *
      *  RTEST(v) can be 0 if and only if (v == Qfalse || v == Qnil).
      */
     return obj & ~RUBY_Qnil;
diff --git a/ruby.c b/ruby.c
index 0a3248f95f..03aeb9b75b 100644
--- a/ruby.c
+++ b/ruby.c
@@ -62,6 +62,9 @@
 #include "ruby/version.h"
 #include "ruby/internal/error.h"
 
+#define singlebit_only_p(x) !((x) & ((x)-1))
+STATIC_ASSERT(Qnil_1bit_from_Qfalse, singlebit_only_p(Qnil^Qfalse));
+
 #ifndef MAXPATHLEN
 # define MAXPATHLEN 1024
 #endif