diff --git a/man/sd_bus_is_open.xml b/man/sd_bus_is_open.xml
index 8e0aed29b3..621ed272bb 100644
--- a/man/sd_bus_is_open.xml
+++ b/man/sd_bus_is_open.xml
@@ -57,6 +57,9 @@
     zero outside of this state, and positive otherwise. Effectively, this function returns positive while regular
     messages can be sent or received on the connection.</para>
 
+    <para>The <parameter>bus</parameter> argument may be <constant>NULL</constant>, zero is also returned in
+    that case.</para>
+
     <para>To be notified when the connection is fully established, use
     <citerefentry><refentrytitle>sd_bus_set_connected_signal</refentrytitle><manvolnum>3</manvolnum></citerefentry> and
     install a match for the <function>Connected()</function> signal on the
@@ -68,8 +71,8 @@
   <refsect1>
     <title>Return Value</title>
 
-    <para>On success, these functions return 0 or a positive integer. On failure, they return a negative errno-style
-    error code.</para>
+    <para>Those functions return 0 if the bus is <emphasis>not</emphasis> in the given state, and a positive
+    integer when it is. On failure, a negative errno-style error code is returned.</para>
 
     <refsect2>
       <title>Errors</title>
diff --git a/src/libsystemd/sd-bus/sd-bus.c b/src/libsystemd/sd-bus/sd-bus.c
index e719c74370..2f4e67c97e 100644
--- a/src/libsystemd/sd-bus/sd-bus.c
+++ b/src/libsystemd/sd-bus/sd-bus.c
@@ -1800,7 +1800,9 @@ void bus_enter_closing(sd_bus *bus) {
 DEFINE_PUBLIC_TRIVIAL_REF_UNREF_FUNC(sd_bus, sd_bus, bus_free);
 
 _public_ int sd_bus_is_open(sd_bus *bus) {
-        assert_return(bus, -EINVAL);
+        if (!bus)
+                return 0;
+
         assert_return(bus = bus_resolve(bus), -ENOPKG);
         assert_return(!bus_pid_changed(bus), -ECHILD);
 
@@ -1808,7 +1810,9 @@ _public_ int sd_bus_is_open(sd_bus *bus) {
 }
 
 _public_ int sd_bus_is_ready(sd_bus *bus) {
-        assert_return(bus, -EINVAL);
+        if (!bus)
+                return 0;
+
         assert_return(bus = bus_resolve(bus), -ENOPKG);
         assert_return(!bus_pid_changed(bus), -ECHILD);