diff --git a/trunk/src/rtmp/srs_protocol_amf0.hpp b/trunk/src/rtmp/srs_protocol_amf0.hpp
index 7ee3f5576..06cceb4c3 100644
--- a/trunk/src/rtmp/srs_protocol_amf0.hpp
+++ b/trunk/src/rtmp/srs_protocol_amf0.hpp
@@ -60,6 +60,10 @@ public:
 	virtual bool is_object_eof();
 	virtual bool is_ecma_array();
 public:
+	/**
+	* get the string of any when is_string() indicates true.
+	* user must ensure the type is a string, or assert failed.
+	*/
 	virtual std::string to_str();
 public:
 	virtual int size() = 0;
diff --git a/trunk/src/utest/srs_utest_amf0.cpp b/trunk/src/utest/srs_utest_amf0.cpp
index a309a85fc..9df0db4be 100644
--- a/trunk/src/utest/srs_utest_amf0.cpp
+++ b/trunk/src/utest/srs_utest_amf0.cpp
@@ -22,6 +22,8 @@ CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE SOFTWARE.
 */
 #include <srs_utest_amf0.hpp>
 
+#include <srs_core_autofree.hpp>
+
 VOID TEST(AMF0Test, Size) 
 {
     // size of elem
@@ -259,3 +261,24 @@ VOID TEST(AMF0Test, Size)
         EXPECT_EQ(size, SrsAmf0Size::array(&o)); 
     }
 }
+
+VOID TEST(AMF0Test, AnyElem) 
+{
+	SrsAmf0Any* o = NULL;
+	
+	// string
+	if (true) {
+		o = SrsAmf0Any::str();
+		SrsAutoFree(SrsAmf0Any, o, false);
+		EXPECT_TRUE(NULL != o);
+		EXPECT_TRUE(o->is_string());
+		EXPECT_STREQ("", o->to_str().c_str());
+	}
+	if (true) {
+		o = SrsAmf0Any::str("winlin");
+		SrsAutoFree(SrsAmf0Any, o, false);
+		EXPECT_TRUE(NULL != o);
+		EXPECT_TRUE(o->is_string());
+		EXPECT_STREQ("winlin", o->to_str().c_str());
+	}
+}