diff --git a/jsonapi.go b/jsonapi.go
index ebd47a17..16fff664 100644
--- a/jsonapi.go
+++ b/jsonapi.go
@@ -262,7 +262,7 @@ func CreateMessage(message string, args ...interface{}) ([]byte, error) {
 		}
 		finalMessage, err = jsonWithArgs(message, args)
 	// Two required strings
-	case "listsinceblock", "setaccount", "signmessage", "walletpassphrase",
+	case "setaccount", "signmessage", "walletpassphrase",
 		"walletpassphrasechange":
 		if len(args) != 2 {
 			err = fmt.Errorf("Missing arguments for %s", message)
@@ -437,6 +437,26 @@ func CreateMessage(message string, args ...interface{}) ([]byte, error) {
 			return finalMessage, err
 		}
 		finalMessage, err = jsonWithArgs(message, args)
+	// Two optional strings
+	case "listsinceblock":
+		if len(args) > 2 {
+			err = fmt.Errorf("Wrong number of arguments for %s", message)
+			return finalMessage, err
+		}
+		ok1 := true
+		ok2 := true
+		if len(args) >= 1 {
+			_, ok1 = args[0].(string)
+		}
+		if len(args) == 2 {
+			_, ok2 = args[1].(string)
+		}
+		if !ok1 || !ok2 {
+			err = fmt.Errorf("Optional arguments must be strings for %s", message)
+			return finalMessage, err
+		}
+		finalMessage, err = jsonWithArgs(message, args)
+
 	// Two required strings, one required float, one optional int,
 	// two optional strings.
 	case "sendfrom":
diff --git a/jsonapi_test.go b/jsonapi_test.go
index d767edfd..66e8a2cc 100644
--- a/jsonapi_test.go
+++ b/jsonapi_test.go
@@ -162,6 +162,11 @@ var cmdtests = []struct {
 	{"signrawtransaction", []interface{}{"hexstring", 1, "test2", "test3", "test4"}, false},
 	{"signrawtransaction", []interface{}{"hexstring", "test", 2, "test3", "test4"}, false},
 	{"signrawtransaction", []interface{}{"hexstring", "test", "test2", 3, "test4"}, false},
+	{"listsinceblock", []interface{}{"test", "test"}, true},
+	{"listsinceblock", []interface{}{"test", "test", "test"}, false},
+	{"listsinceblock", []interface{}{"test"}, true},
+	{"listsinceblock", []interface{}{}, true},
+	{"listsinceblock", []interface{}{1, "test"}, false},
 	{"fakecommand", nil, false},
 }