diff --git a/sml/src/sml_octet_string.c b/sml/src/sml_octet_string.c
index 9a43787..ffae7ea 100644
--- a/sml/src/sml_octet_string.c
+++ b/sml/src/sml_octet_string.c
@@ -84,6 +84,11 @@ octet_string *sml_octet_string_parse(sml_buffer *buf) {
 }
 
 void sml_octet_string_write(octet_string *str, sml_buffer *buf) {
+	if (str == 0) {
+		sml_buf_optional_write(buf);
+		return;
+	}
+	
     sml_buf_set_type_and_length(buf, SML_TYPE_OCTET_STRING, (unsigned int) str->len);
     memcpy(sml_buf_get_current_buf(buf), str->str, str->len);
     buf->cursor += str->len;
diff --git a/test/src/sml_octet_string_test.c b/test/src/sml_octet_string_test.c
index c6b0fbd..5e8e61c 100644
--- a/test/src/sml_octet_string_test.c
+++ b/test/src/sml_octet_string_test.c
@@ -23,12 +23,20 @@
 TEST_GROUP(sml_octet_string);
 
 sml_buffer *buf;
+
 void expected_octet_string(octet_string *str, char *content, int len) {
 	TEST_ASSERT_NOT_NULL(str);
 	TEST_ASSERT_EQUAL(len, str->len);
 	TEST_ASSERT_EQUAL_MEMORY(content, str->str, len);
 }
 
+void expected_buf(char *hex, int len) {
+	unsigned char expected_buf[len];
+	hex2binary(hex, expected_buf);
+	TEST_ASSERT_EQUAL_MEMORY(expected_buf, buf->buffer, len);
+	TEST_ASSERT_EQUAL(len, buf->cursor);
+}
+
 TEST_SETUP(sml_octet_string) {
 	buf = sml_buffer_init(512);
 }
@@ -64,11 +72,24 @@ TEST(sml_octet_string, parse_optional) {
 	TEST_ASSERT_EQUAL(1, buf->cursor);
 }
 
+TEST(sml_octet_string, write) {
+	octet_string *str = sml_octet_string_init((unsigned char *)"Hallo", 5);
+	sml_octet_string_write(str, buf);
+	expected_buf("0648616C6C6F", 6);
+}
+
+TEST(sml_octet_string, write_optional) {
+	sml_octet_string_write(0, buf);
+	expected_buf("01", 1);
+}
+
 TEST_GROUP_RUNNER(sml_octet_string) {
 	RUN_TEST_CASE(sml_octet_string, init);
 	RUN_TEST_CASE(sml_octet_string, parse);
 	RUN_TEST_CASE(sml_octet_string, parse_multiple_tl_fields);
 	RUN_TEST_CASE(sml_octet_string, parse_optional);
+	RUN_TEST_CASE(sml_octet_string, write);
+	RUN_TEST_CASE(sml_octet_string, write_optional);
 }