diff --git a/class.c b/class.c
index 955059d777..53736346b7 100644
--- a/class.c
+++ b/class.c
@@ -1578,6 +1578,9 @@ singleton_class_of(VALUE obj)
 	switch (BUILTIN_TYPE(obj)) {
 	  case T_FLOAT: case T_BIGNUM: case T_SYMBOL:
 	    goto no_singleton;
+	  case T_STRING:
+	    if (FL_TEST_RAW(obj, RSTRING_FSTR)) goto no_singleton;
+	    break;
 	}
     }
 
diff --git a/test/-ext-/string/test_fstring.rb b/test/-ext-/string/test_fstring.rb
index 39ed020892..1b3b15c922 100644
--- a/test/-ext-/string/test_fstring.rb
+++ b/test/-ext-/string/test_fstring.rb
@@ -1,8 +1,11 @@
 # frozen_string_literal: false
 require 'test/unit'
 require '-test-/string'
+require_relative '../symbol/noninterned_name'
 
 class Test_String_Fstring < Test::Unit::TestCase
+  include Test_Symbol::NonInterned
+
   def assert_fstring(str)
     fstr = Bug::String.fstring(str)
     yield str
@@ -54,6 +57,12 @@ class Test_String_Fstring < Test::Unit::TestCase
     assert_fstring(str) {|s| assert_send([s, :respond_to?, :foo])}
   end
 
+  def test_singleton_class
+    str = noninterned_name.force_encoding("us-ascii")
+    fstr = Bug::String.fstring(str)
+    assert_raise(TypeError) {fstr.singleton_class}
+  end
+
   class S < String
   end
 
diff --git a/test/ruby/test_eval.rb b/test/ruby/test_eval.rb
index 6982429b51..bf0a63171d 100644
--- a/test/ruby/test_eval.rb
+++ b/test/ruby/test_eval.rb
@@ -504,6 +504,14 @@ class TestEval < Test::Unit::TestCase
     assert_same a, b
   end
 
+  def test_fstring_instance_eval
+    bug = "[ruby-core:78116] [Bug #12930]".freeze
+    assert_same bug, (bug.instance_eval {self})
+    assert_raise(RuntimeError) {
+      bug.instance_eval {@ivar = true}
+    }
+  end
+
   def test_gced_binding_block
     assert_normal_exit %q{
       def m
diff --git a/version.h b/version.h
index b9028f3146..2db791e3a9 100644
--- a/version.h
+++ b/version.h
@@ -1,6 +1,6 @@
 #define RUBY_VERSION "2.3.3"
 #define RUBY_RELEASE_DATE "2017-03-27"
-#define RUBY_PATCHLEVEL 267
+#define RUBY_PATCHLEVEL 268
 
 #define RUBY_RELEASE_YEAR 2017
 #define RUBY_RELEASE_MONTH 3
diff --git a/vm_eval.c b/vm_eval.c
index d275b8e6e8..138cc2fd54 100644
--- a/vm_eval.c
+++ b/vm_eval.c
@@ -1667,6 +1667,8 @@ singleton_class_for_eval(VALUE self)
     switch (BUILTIN_TYPE(self)) {
       case T_FLOAT: case T_BIGNUM: case T_SYMBOL:
 	return Qnil;
+      case T_STRING:
+	if (FL_TEST_RAW(self, RSTRING_FSTR)) return Qnil;
       default:
 	return rb_singleton_class(self);
     }