diff --git a/src/basic/mountpoint-util.c b/src/basic/mountpoint-util.c
index 0c9fc0a350..1136454b7d 100644
--- a/src/basic/mountpoint-util.c
+++ b/src/basic/mountpoint-util.c
@@ -598,3 +598,7 @@ int mount_propagation_flag_from_string(const char *name, unsigned long *ret) {
                 return -EINVAL;
         return 0;
 }
+
+bool mount_propagation_flag_is_valid(unsigned long flag) {
+        return IN_SET(flag, 0, MS_SHARED, MS_PRIVATE, MS_SLAVE);
+}
diff --git a/src/basic/mountpoint-util.h b/src/basic/mountpoint-util.h
index c7b24584da..19709195e8 100644
--- a/src/basic/mountpoint-util.h
+++ b/src/basic/mountpoint-util.h
@@ -57,3 +57,4 @@ int mount_nofollow(const char *source, const char *target, const char *filesyste
 
 const char *mount_propagation_flag_to_string(unsigned long flags);
 int mount_propagation_flag_from_string(const char *name, unsigned long *ret);
+bool mount_propagation_flag_is_valid(unsigned long flag);
diff --git a/src/core/dbus-execute.c b/src/core/dbus-execute.c
index 4b41cfe778..41eeb1ee94 100644
--- a/src/core/dbus-execute.c
+++ b/src/core/dbus-execute.c
@@ -1600,7 +1600,7 @@ static int parse_personality(const char *s, unsigned long *p) {
 }
 
 static const char* mount_propagation_flag_to_string_with_check(unsigned long n) {
-        if (!IN_SET(n, 0, MS_SHARED, MS_PRIVATE, MS_SLAVE))
+        if (!mount_propagation_flag_is_valid(n))
                 return NULL;
 
         return mount_propagation_flag_to_string(n);