diff --git a/Makefile.am b/Makefile.am
index f7bc5f36cc..4db064e7c2 100644
--- a/Makefile.am
+++ b/Makefile.am
@@ -3836,6 +3836,13 @@ test_login_LDADD = \
 	libsystemd-login-internal.la \
 	libsystemd-shared.la
 
+test_login_shared_SOURCES = \
+	src/login/test-login-shared.c
+
+test_login_shared_LDADD = \
+	libsystemd-login-internal.la \
+	libsystemd-shared.la
+
 test_inhibit_SOURCES = \
 	src/login/test-inhibit.c
 
@@ -3862,7 +3869,8 @@ manual_tests += \
 	test-inhibit
 
 tests += \
-	test-login-tables
+	test-login-tables \
+	test-login-shared
 
 libsystemd_login_la_SOURCES = \
 	src/login/sd-login.c \
diff --git a/src/login/login-shared.c b/src/login/login-shared.c
index ff13c28861..054c77503b 100644
--- a/src/login/login-shared.c
+++ b/src/login/login-shared.c
@@ -1,8 +1,29 @@
+/*-*- Mode: C; c-basic-offset: 8; indent-tabs-mode: nil -*-*/
+
+/***
+  This file is part of systemd.
+
+  Copyright 2013 Zbigniew Jędrzejewski-Szmek
+
+  systemd is free software; you can redistribute it and/or modify it
+  under the terms of the GNU Lesser General Public License as published by
+  the Free Software Foundation; either version 2.1 of the License, or
+  (at your option) any later version.
+
+  systemd is distributed in the hope that it will be useful, but
+  WITHOUT ANY WARRANTY; without even the implied warranty of
+  MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU
+  Lesser General Public License for more details.
+
+  You should have received a copy of the GNU Lesser General Public License
+  along with systemd; If not, see <http://www.gnu.org/licenses/>.
+***/
+
 #include "login-shared.h"
 #include "def.h"
 
 bool session_id_valid(const char *id) {
         assert(id);
 
-        return id + strspn(id, LETTERS DIGITS) == '\0';
+        return id[0] && id[strspn(id, LETTERS DIGITS)] == '\0';
 }
diff --git a/src/login/login-shared.h b/src/login/login-shared.h
index 728ef0038f..b2787c9c62 100644
--- a/src/login/login-shared.h
+++ b/src/login/login-shared.h
@@ -1,3 +1,24 @@
+/*-*- Mode: C; c-basic-offset: 8; indent-tabs-mode: nil -*-*/
+
+/***
+  This file is part of systemd.
+
+  Copyright 2013 Zbigniew Jędrzejewski-Szmek
+
+  systemd is free software; you can redistribute it and/or modify it
+  under the terms of the GNU Lesser General Public License as published by
+  the Free Software Foundation; either version 2.1 of the License, or
+  (at your option) any later version.
+
+  systemd is distributed in the hope that it will be useful, but
+  WITHOUT ANY WARRANTY; without even the implied warranty of
+  MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU
+  Lesser General Public License for more details.
+
+  You should have received a copy of the GNU Lesser General Public License
+  along with systemd; If not, see <http://www.gnu.org/licenses/>.
+***/
+
 #include <stdbool.h>
 
 bool session_id_valid(const char *id);
diff --git a/src/login/test-login-shared.c b/src/login/test-login-shared.c
new file mode 100644
index 0000000000..2df60292bf
--- /dev/null
+++ b/src/login/test-login-shared.c
@@ -0,0 +1,38 @@
+/*-*- Mode: C; c-basic-offset: 8; indent-tabs-mode: nil -*-*/
+
+/***
+  This file is part of systemd.
+
+  Copyright 2013 Zbigniew Jędrzejewski-Szmek
+
+  systemd is free software; you can redistribute it and/or modify it
+  under the terms of the GNU Lesser General Public License as published by
+  the Free Software Foundation; either version 2.1 of the License, or
+  (at your option) any later version.
+
+  systemd is distributed in the hope that it will be useful, but
+  WITHOUT ANY WARRANTY; without even the implied warranty of
+  MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU
+  Lesser General Public License for more details.
+
+  You should have received a copy of the GNU Lesser General Public License
+  along with systemd; If not, see <http://www.gnu.org/licenses/>.
+***/
+
+#include "macro.h"
+#include "login-shared.h"
+
+static void test_session_id_valid(void) {
+        assert_se(session_id_valid("c1"));
+        assert_se(session_id_valid("1234"));
+
+        assert_se(!session_id_valid("1-2"));
+        assert_se(!session_id_valid(""));
+        assert_se(!session_id_valid("\tid"));
+}
+
+int main(int argc, char* argv[]) {
+        test_session_id_valid();
+
+        return 0;
+}