diff --git a/ext/stringio/stringio.c b/ext/stringio/stringio.c
index f581cd5..1ce9003 100644
--- a/ext/stringio/stringio.c
+++ b/ext/stringio/stringio.c
@@ -48,7 +48,7 @@ static long strio_write(VALUE self, VALUE str);
 
 #define IS_STRIO(obj) (rb_typeddata_is_kind_of((obj), &strio_data_type))
 #define error_inval(msg) (rb_syserr_fail(EINVAL, msg))
-#define get_enc(ptr) ((ptr)->enc ? (ptr)->enc : rb_enc_get((ptr)->string))
+#define get_enc(ptr) ((ptr)->enc ? (ptr)->enc : !NIL_P((ptr)->string) ? rb_enc_get((ptr)->string) : NULL)
 
 static struct StringIO *
 strio_alloc(void)
@@ -281,13 +281,13 @@ strio_init(int argc, VALUE *argv, struct StringIO *ptr, VALUE self)
 
     argc = rb_scan_args(argc, argv, "02:", &string, &vmode, &opt);
     rb_io_extract_modeenc(&vmode, 0, opt, &oflags, &ptr->flags, &convconfig);
-    if (argc) {
+    if (!NIL_P(string)) {
 	StringValue(string);
     }
-    else {
+    else if (!argc) {
 	string = rb_enc_str_new("", 0, rb_default_external_encoding());
     }
-    if (OBJ_FROZEN_RAW(string)) {
+    if (!NIL_P(string) && OBJ_FROZEN_RAW(string)) {
 	if (ptr->flags & FMODE_WRITABLE) {
 	    rb_syserr_fail(EACCES, 0);
 	}
@@ -297,11 +297,11 @@ strio_init(int argc, VALUE *argv, struct StringIO *ptr, VALUE self)
 	    ptr->flags |= FMODE_WRITABLE;
 	}
     }
-    if (ptr->flags & FMODE_TRUNC) {
+    if (!NIL_P(string) && (ptr->flags & FMODE_TRUNC)) {
 	rb_str_resize(string, 0);
     }
     RB_OBJ_WRITE(self, &ptr->string, string);
-    if (argc == 1) {
+    if (argc == 1 && !NIL_P(string)) {
 	ptr->enc = rb_enc_get(string);
     }
     else {
@@ -595,6 +595,7 @@ static struct StringIO *
 strio_to_read(VALUE self)
 {
     struct StringIO *ptr = readable(self);
+    if (NIL_P(ptr->string)) return NULL;
     if (ptr->pos < RSTRING_LEN(ptr->string)) return ptr;
     return NULL;
 }
@@ -872,7 +873,7 @@ strio_getc(VALUE self)
     int len;
     char *p;
 
-    if (pos >= RSTRING_LEN(str)) {
+    if (NIL_P(str) || pos >= RSTRING_LEN(str)) {
 	return Qnil;
     }
     p = RSTRING_PTR(str)+pos;
@@ -893,7 +894,7 @@ strio_getbyte(VALUE self)
 {
     struct StringIO *ptr = readable(self);
     int c;
-    if (ptr->pos >= RSTRING_LEN(ptr->string)) {
+    if (NIL_P(ptr->string) || ptr->pos >= RSTRING_LEN(ptr->string)) {
 	return Qnil;
     }
     c = RSTRING_PTR(ptr->string)[ptr->pos++];
@@ -931,6 +932,7 @@ strio_ungetc(VALUE self, VALUE c)
     rb_encoding *enc, *enc2;
 
     check_modifiable(ptr);
+    if (NIL_P(ptr->string)) return Qnil;
     if (NIL_P(c)) return Qnil;
     if (RB_INTEGER_TYPE_P(c)) {
 	int len, cc = NUM2INT(c);
@@ -968,6 +970,7 @@ strio_ungetbyte(VALUE self, VALUE c)
     struct StringIO *ptr = readable(self);
 
     check_modifiable(ptr);
+    if (NIL_P(ptr->string)) return Qnil;
     if (NIL_P(c)) return Qnil;
     if (RB_INTEGER_TYPE_P(c)) {
         /* rb_int_and() not visible from exts */
@@ -1171,7 +1174,7 @@ prepare_getline_args(struct StringIO *ptr, struct getline_arg *arg, int argc, VA
 	if (!NIL_P(lim)) limit = NUM2LONG(lim);
 	break;
     }
-    if (!NIL_P(rs)) {
+    if (!NIL_P(ptr->string) && !NIL_P(rs)) {
         rb_encoding *enc_rs, *enc_io;
         enc_rs = rb_enc_get(rs);
         enc_io = get_enc(ptr);
@@ -1226,7 +1229,7 @@ strio_getline(struct getline_arg *arg, struct StringIO *ptr)
     long w = 0;
     rb_encoding *enc = get_enc(ptr);
 
-    if (ptr->pos >= (n = RSTRING_LEN(ptr->string))) {
+    if (NIL_P(ptr->string) || ptr->pos >= (n = RSTRING_LEN(ptr->string))) {
 	return Qnil;
     }
     s = RSTRING_PTR(ptr->string);
@@ -1323,6 +1326,7 @@ strio_gets(int argc, VALUE *argv, VALUE self)
     VALUE str;
 
     if (prepare_getline_args(ptr, &arg, argc, argv)->limit == 0) {
+	if (NIL_P(ptr->string)) return Qnil;
 	return rb_enc_str_new(0, 0, get_enc(ptr));
     }
 
@@ -1437,6 +1441,7 @@ strio_write(VALUE self, VALUE str)
     if (!RB_TYPE_P(str, T_STRING))
 	str = rb_obj_as_string(str);
     enc = get_enc(ptr);
+    if (!enc) return 0;
     enc2 = rb_enc_get(str);
     if (enc != enc2 && enc != ascii8bit && enc != (usascii = rb_usascii_encoding())) {
 	VALUE converted = rb_str_conv_enc(str, enc2, enc);
@@ -1509,10 +1514,12 @@ strio_putc(VALUE self, VALUE ch)
 
     check_modifiable(ptr);
     if (RB_TYPE_P(ch, T_STRING)) {
+	if (NIL_P(ptr->string)) return ch;
 	str = rb_str_substr(ch, 0, 1);
     }
     else {
 	char c = NUM2CHR(ch);
+	if (NIL_P(ptr->string)) return ch;
 	str = rb_str_new(&c, 1);
     }
     strio_write(self, str);
@@ -1555,7 +1562,8 @@ strio_read(int argc, VALUE *argv, VALUE self)
 	    if (len < 0) {
 		rb_raise(rb_eArgError, "negative length %ld given", len);
 	    }
-	    if (len > 0 && ptr->pos >= RSTRING_LEN(ptr->string)) {
+	    if (len > 0 &&
+		(NIL_P(ptr->string) || ptr->pos >= RSTRING_LEN(ptr->string))) {
 		if (!NIL_P(str)) rb_str_resize(str, 0);
 		return Qnil;
 	    }
@@ -1564,6 +1572,7 @@ strio_read(int argc, VALUE *argv, VALUE self)
 	}
 	/* fall through */
       case 0:
+	if (NIL_P(ptr->string)) return Qnil;
 	len = RSTRING_LEN(ptr->string);
 	if (len <= ptr->pos) {
 	    rb_encoding *enc = get_enc(ptr);
@@ -1733,7 +1742,7 @@ strio_size(VALUE self)
 {
     VALUE string = StringIO(self)->string;
     if (NIL_P(string)) {
-	rb_raise(rb_eIOError, "not opened");
+	return INT2FIX(0);
     }
     return ULONG2NUM(RSTRING_LEN(string));
 }
@@ -1750,10 +1759,12 @@ strio_truncate(VALUE self, VALUE len)
 {
     VALUE string = writable(self)->string;
     long l = NUM2LONG(len);
-    long plen = RSTRING_LEN(string);
+    long plen;
     if (l < 0) {
 	error_inval("negative length");
     }
+    if (NIL_P(string)) return 0;
+    plen = RSTRING_LEN(string);
     rb_str_resize(string, l);
     if (plen < l) {
 	MEMZERO(RSTRING_PTR(string) + plen, char, l - plen);
@@ -1824,7 +1835,7 @@ strio_set_encoding(int argc, VALUE *argv, VALUE self)
 	}
     }
     ptr->enc = enc;
-    if (WRITABLE(self)) {
+    if (!NIL_P(ptr->string) && WRITABLE(self)) {
 	rb_enc_associate(ptr->string, enc);
     }
 
diff --git a/test/stringio/test_stringio.rb b/test/stringio/test_stringio.rb
index b0eff57..2af6923 100644
--- a/test/stringio/test_stringio.rb
+++ b/test/stringio/test_stringio.rb
@@ -22,10 +22,11 @@ def test_initialize
     assert_kind_of StringIO, StringIO.new
     assert_kind_of StringIO, StringIO.new('str')
     assert_kind_of StringIO, StringIO.new('str', 'r+')
+    assert_kind_of StringIO, StringIO.new(nil)
     assert_raise(ArgumentError) { StringIO.new('', 'x') }
     assert_raise(ArgumentError) { StringIO.new('', 'rx') }
     assert_raise(ArgumentError) { StringIO.new('', 'rbt') }
-    assert_raise(TypeError) { StringIO.new(nil) }
+    assert_raise(TypeError) { StringIO.new(Object) }
 
     o = Object.new
     def o.to_str
@@ -40,6 +41,13 @@ def o.to_str
     assert_kind_of StringIO, StringIO.new(o)
   end
 
+  def test_null
+    io = StringIO.new(nil)
+    assert_nil io.gets
+    io.puts "abc"
+    assert_nil io.string
+  end
+
   def test_truncate
     io = StringIO.new("")
     io.puts "abc"