diff --git a/man/systemd.unit.xml b/man/systemd.unit.xml
index d603ec9744..fcd1f914a8 100644
--- a/man/systemd.unit.xml
+++ b/man/systemd.unit.xml
@@ -856,16 +856,18 @@
         <term><varname>JoinsNamespaceOf=</varname></term>
 
         <listitem><para>For units that start processes (such as service units), lists one or more other units
-        whose network and/or temporary file namespace to join. This only applies to unit types which support
-        the <varname>PrivateNetwork=</varname>, <varname>NetworkNamespacePath=</varname>,
+        whose network and/or temporary file namespace to join. If this is specified on a unit (say, a.service
+        has <varname>JoinsNamespaceOf=b.service</varname>), then this the inverse dependency
+        (<varname>JoinsNamespaceOf=a.service</varname> for b.service) is implied. This only applies to unit
+        types which support the <varname>PrivateNetwork=</varname>, <varname>NetworkNamespacePath=</varname>,
         <varname>PrivateIPC=</varname>, <varname>IPCNamespacePath=</varname>, and
         <varname>PrivateTmp=</varname> directives (see
         <citerefentry><refentrytitle>systemd.exec</refentrytitle><manvolnum>5</manvolnum></citerefentry> for
         details). If a unit that has this setting set is started, its processes will see the same
         <filename>/tmp/</filename>, <filename>/var/tmp/</filename>, IPC namespace and network namespace as
-        one listed unit that is started. If multiple listed units are already started, it is not defined
-        which namespace is joined. Note that this setting only has an effect if
-        <varname>PrivateNetwork=</varname>/<varname>NetworkNamespacePath=</varname>,
+        one listed unit that is started. If multiple listed units are already started and these do not share
+        their namespace, then it is not defined which namespace is joined. Note that this setting only has an
+        effect if <varname>PrivateNetwork=</varname>/<varname>NetworkNamespacePath=</varname>,
         <varname>PrivateIPC=</varname>/<varname>IPCNamespacePath=</varname> and/or
         <varname>PrivateTmp=</varname> is enabled for both the unit that joins the namespace and the unit
         whose namespace is joined.</para></listitem>
diff --git a/src/core/unit.c b/src/core/unit.c
index 7a43355832..be57bdbd1d 100644
--- a/src/core/unit.c
+++ b/src/core/unit.c
@@ -3209,12 +3209,11 @@ int unit_add_dependency(
                 return r;
         notify = r > 0;
 
-        if (inverse_table[d] != _UNIT_DEPENDENCY_INVALID && inverse_table[d] != d) {
-                r = unit_add_dependency_hashmap(&other->dependencies, inverse_table[d], u, 0, mask);
-                if (r < 0)
-                        return r;
-                notify_other = r > 0;
-        }
+        assert(inverse_table[d] >= 0 && inverse_table[d] < _UNIT_DEPENDENCY_MAX);
+        r = unit_add_dependency_hashmap(&other->dependencies, inverse_table[d], u, 0, mask);
+        if (r < 0)
+                return r;
+        notify_other = r > 0;
 
         if (add_reference) {
                 r = unit_add_dependency_hashmap(&u->dependencies, UNIT_REFERENCES, other, mask, 0);
diff --git a/test/testsuite-23.units/testsuite-23-joins-namespace-of-5.service b/test/testsuite-23.units/testsuite-23-joins-namespace-of-5.service
index 80594ccba2..c3d316bfa2 100644
--- a/test/testsuite-23.units/testsuite-23-joins-namespace-of-5.service
+++ b/test/testsuite-23.units/testsuite-23-joins-namespace-of-5.service
@@ -3,4 +3,4 @@
 Type=oneshot
 MountAPIVFS=yes
 PrivateTmp=yes
-ExecStart=test ! -e /tmp/shared-private-file
+ExecStart=test -e /tmp/shared-private-file
diff --git a/test/testsuite-23.units/testsuite-23-joins-namespace-of-8.service b/test/testsuite-23.units/testsuite-23-joins-namespace-of-8.service
index f3ec0668de..42053b99f8 100644
--- a/test/testsuite-23.units/testsuite-23-joins-namespace-of-8.service
+++ b/test/testsuite-23.units/testsuite-23-joins-namespace-of-8.service
@@ -4,6 +4,6 @@ Type=notify
 NotifyAccess=all
 MountAPIVFS=yes
 PrivateTmp=yes
-ExecStartPre=test ! -e /tmp/shared-private-file-x
+ExecStartPre=test -e /tmp/shared-private-file-x
 ExecStartPre=test ! -e /tmp/hoge
 ExecStart=/bin/bash -c 'touch /tmp/shared-private-file-y && systemd-notify --ready && sleep infinity'
diff --git a/test/testsuite-23.units/testsuite-23-joins-namespace-of-9.service b/test/testsuite-23.units/testsuite-23-joins-namespace-of-9.service
index 01de7f9054..a50a7fcdc2 100644
--- a/test/testsuite-23.units/testsuite-23-joins-namespace-of-9.service
+++ b/test/testsuite-23.units/testsuite-23-joins-namespace-of-9.service
@@ -7,5 +7,5 @@ Type=oneshot
 MountAPIVFS=yes
 PrivateTmp=yes
 ExecStart=test ! -e /tmp/shared-private-file-x
-ExecStart=test -e /tmp/shared-private-file-y
+ExecStart=test ! -e /tmp/shared-private-file-y
 ExecStart=test ! -e /tmp/hoge