diff --git a/src/libsystemd/sd-bus/bus-message.c b/src/libsystemd/sd-bus/bus-message.c
index a2464e1a46..427d42f296 100644
--- a/src/libsystemd/sd-bus/bus-message.c
+++ b/src/libsystemd/sd-bus/bus-message.c
@@ -284,7 +284,7 @@ static int message_append_field_signature(
         /* dbus1 doesn't allow signatures over 8bit, let's enforce
          * this globally, to not risk convertability */
         l = strlen(s);
-        if (l > 255)
+        if (l > SD_BUS_MAXIMUM_SIGNATURE_LENGTH)
                 return -EINVAL;
 
         /* Signature "(yv)" where the variant contains "g" */
diff --git a/src/libsystemd/sd-bus/bus-signature.c b/src/libsystemd/sd-bus/bus-signature.c
index 1ecd6e8b7e..b420ba3688 100644
--- a/src/libsystemd/sd-bus/bus-signature.c
+++ b/src/libsystemd/sd-bus/bus-signature.c
@@ -144,5 +144,5 @@ bool signature_is_valid(const char *s, bool allow_dict_entry) {
                 p += t;
         }
 
-        return p - s <= 255;
+        return p - s <= SD_BUS_MAXIMUM_SIGNATURE_LENGTH;
 }
diff --git a/src/systemd/sd-bus.h b/src/systemd/sd-bus.h
index 129cc93328..311602d048 100644
--- a/src/systemd/sd-bus.h
+++ b/src/systemd/sd-bus.h
@@ -33,6 +33,9 @@ _SD_BEGIN_DECLARATIONS;
 #define SD_BUS_DEFAULT_USER ((sd_bus *) 2)
 #define SD_BUS_DEFAULT_SYSTEM ((sd_bus *) 3)
 
+/* https://dbus.freedesktop.org/doc/dbus-specification.html#message-protocol-marshaling-signature */
+#define SD_BUS_MAXIMUM_SIGNATURE_LENGTH 255
+
 /* Types */
 
 typedef struct sd_bus sd_bus;