diff --git a/io.c b/io.c
index 1254e92363..6226108ad6 100644
--- a/io.c
+++ b/io.c
@@ -12224,10 +12224,10 @@ argf_inplace_mode_set(VALUE argf, VALUE val)
 	ARGF.inplace = 0;
     }
     else {
-	StringValue(val);
+	const char *suffix = StringValueCStr(val);
 	if (ARGF.inplace) free(ARGF.inplace);
 	ARGF.inplace = 0;
-	ARGF.inplace = strdup(RSTRING_PTR(val));
+	ARGF.inplace = strdup(suffix);
     }
     return argf;
 }
diff --git a/test/ruby/test_argf.rb b/test/ruby/test_argf.rb
index bc5c67d0d8..a112cf3e84 100644
--- a/test/ruby/test_argf.rb
+++ b/test/ruby/test_argf.rb
@@ -310,6 +310,12 @@ class TestArgf < Test::Unit::TestCase
     };
   end
 
+  def test_inplace_invalid_backup
+    assert_raise(ArgumentError, '[ruby-dev:50272] [Bug #13960]') {
+      ARGF.inplace_mode = "a\0"
+    }
+  end
+
   def test_encoding
     ruby('-e', "#{<<~"{#"}\n#{<<~'};'}", @t1.path, @t2.path, @t3.path) do |f|
       {#