diff --git a/spec/core/marshal/dump_spec.rb b/spec/core/marshal/dump_spec.rb
index 25b08ae94..5657a227e 100644
--- a/spec/core/marshal/dump_spec.rb
+++ b/spec/core/marshal/dump_spec.rb
@@ -321,21 +321,15 @@ def _dump(level)
 
   describe "with a String" do
     it "dumps a blank String" do
-      NATFIXME 'dumps a blank String', exception: SpecFailedException do
-        Marshal.dump("".dup.force_encoding("binary")).should == "\004\b\"\000"
-      end
+      Marshal.dump("".dup.force_encoding("binary")).should == "\004\b\"\000"
     end
 
     it "dumps a short String" do
-      NATFIXME 'dumps a short String', exception: SpecFailedException do
-        Marshal.dump("short".dup.force_encoding("binary")).should == "\004\b\"\012short"
-      end
+      Marshal.dump("short".dup.force_encoding("binary")).should == "\004\b\"\012short"
     end
 
     it "dumps a long String" do
-      NATFIXME 'dumps a long String', exception: SpecFailedException do
-        Marshal.dump(("big" * 100).force_encoding("binary")).should == "\004\b\"\002,\001#{"big" * 100}"
-      end
+      Marshal.dump(("big" * 100).force_encoding("binary")).should == "\004\b\"\002,\001#{"big" * 100}"
     end
 
     it "dumps a String extended with a Module" do
diff --git a/spec/core/marshal/shared/load.rb b/spec/core/marshal/shared/load.rb
index 253935947..379332edd 100644
--- a/spec/core/marshal/shared/load.rb
+++ b/spec/core/marshal/shared/load.rb
@@ -484,9 +484,7 @@
     arr = ArraySubPush.new
     arr[0] = '1'
     arr_dump = Marshal.dump(arr)
-    NATFIXME 'loads subclasses of Array with overridden << and push correctly', exception: ArgumentError, message: 'marshal data too short' do
-      Marshal.send(@method, arr_dump).should == arr
-    end
+    Marshal.send(@method, arr_dump).should == arr
   end
 
   it "raises a TypeError with bad Marshal version" do
@@ -612,7 +610,7 @@
       h = { key: s }
       h.instance_variable_set :@hash_ivar, 'hash ivar'
 
-      NATFIXME 'preserves hash ivars when hash contains a string having ivar', exception: ArgumentError, message: 'marshal data too short' do
+      NATFIXME 'preserves hash ivars when hash contains a string having ivar', exception: SpecFailedException do
         unmarshalled = Marshal.send(@method, Marshal.dump(h))
         unmarshalled.instance_variable_get(:@hash_ivar).should == 'hash ivar'
         unmarshalled[:key].instance_variable_get(:@string_ivar).should == 'string ivar'
@@ -746,9 +744,7 @@
     it "loads a string through StringIO stream" do
       require 'stringio'
       obj = "This is a string which should be unmarshalled through StringIO stream!"
-      NATFIXME 'loads a string through StringIO stream', exception: ArgumentError, message: 'marshal data too short' do
-        Marshal.send(@method, StringIO.new(Marshal.dump(obj))).should == obj
-      end
+      Marshal.send(@method, StringIO.new(Marshal.dump(obj))).should == obj
     end
 
     it "sets binmode if it is loading through StringIO stream" do
@@ -810,13 +806,11 @@ def io.binmode; raise "binmode"; end
     end
 
     it "raises ArgumentError when end of byte sequence reached before string characters end" do
-      NATFIXME 'raises ArgumentError when end of byte sequence reached before string characters end', exception: SpecFailedException do
-        Marshal.dump("hello").should == "\x04\b\"\nhello"
+      Marshal.dump("hello").should == "\x04\b\"\nhello"
 
-        -> {
-          Marshal.send(@method, "\x04\b\"\nhel")
-        }.should raise_error(ArgumentError, "marshal data too short")
-      end
+      -> {
+        Marshal.send(@method, "\x04\b\"\nhel")
+      }.should raise_error(ArgumentError, "marshal data too short")
     end
   end
 
diff --git a/src/marshal.rb b/src/marshal.rb
index cebffccec..5b01806c9 100644
--- a/src/marshal.rb
+++ b/src/marshal.rb
@@ -140,10 +140,10 @@ def write_integer(value)
     end
 
     def write_string(value)
-      write_char('I')
+      write_char('I') if value.encoding != Encoding::ASCII_8BIT
       write_char('"')
       write_string_bytes(value)
-      write_encoding_bytes(value)
+      write_encoding_bytes(value) if value.encoding != Encoding::ASCII_8BIT
     end
 
     def write_symbol(value)