diff --git a/src/core/unit.c b/src/core/unit.c
index 64ab49dc4a..a43f1d7785 100644
--- a/src/core/unit.c
+++ b/src/core/unit.c
@@ -2441,6 +2441,9 @@ int unit_set_slice(Unit *u, Unit *slice) {
         if (u->type == UNIT_SLICE)
                 return -EINVAL;
 
+        if (unit_active_state(u) != UNIT_INACTIVE)
+                return -EBUSY;
+
         if (slice->type != UNIT_SLICE)
                 return -EINVAL;