diff --git a/eval_intern.h b/eval_intern.h
index 34489777a2..9fa9031189 100644
--- a/eval_intern.h
+++ b/eval_intern.h
@@ -302,7 +302,16 @@ VALUE rb_ec_backtrace_location_ary(const rb_execution_context_t *ec, long lev, l
 
 #ifndef CharNext		/* defined as CharNext[AW] on Windows. */
 # ifdef HAVE_MBLEN
-#  define CharNext(p) ((p) + mblen((p), RUBY_MBCHAR_MAXSIZE))
+#  define CharNext(p) rb_char_next(p)
+static inline const char *
+rb_char_next(const char *p)
+{
+    if (p) {
+        int len = mblen(p, RUBY_MBCHAR_MAXSIZE);
+        p += len > 0 ? len : 1;
+    }
+    return p;
+}
 # else
 #  define CharNext(p) ((p) + 1)
 # endif
diff --git a/test/ruby/test_rubyoptions.rb b/test/ruby/test_rubyoptions.rb
index b7831948f0..0d0f4b4d7e 100644
--- a/test/ruby/test_rubyoptions.rb
+++ b/test/ruby/test_rubyoptions.rb
@@ -1075,6 +1075,11 @@ class TestRubyOptions < Test::Unit::TestCase
     end
   end
 
+  def test_rubylib_invalid_encoding
+    env = {"RUBYLIB"=>"\xFF", "LOCALE"=>"en_US.UTF-8", "LC_ALL"=>"en_US.UTF-8"}
+    assert_ruby_status([env, "-e;"])
+  end
+
   def test_null_script
     skip "#{IO::NULL} is not a character device" unless File.chardev?(IO::NULL)
     assert_in_out_err([IO::NULL], success: true)
diff --git a/version.h b/version.h
index 5f07f2c35e..15531eb1ce 100644
--- a/version.h
+++ b/version.h
@@ -12,7 +12,7 @@
 # define RUBY_VERSION_MINOR RUBY_API_VERSION_MINOR
 #define RUBY_VERSION_TEENY 0
 #define RUBY_RELEASE_DATE RUBY_RELEASE_YEAR_STR"-"RUBY_RELEASE_MONTH_STR"-"RUBY_RELEASE_DAY_STR
-#define RUBY_PATCHLEVEL 58
+#define RUBY_PATCHLEVEL 59
 
 #define RUBY_RELEASE_YEAR 2021
 #define RUBY_RELEASE_MONTH 4