diff --git a/server/src/main/java/org/elasticsearch/cluster/block/ClusterBlocks.java b/server/src/main/java/org/elasticsearch/cluster/block/ClusterBlocks.java
index 736f7f9379efd..264479cad49e8 100644
--- a/server/src/main/java/org/elasticsearch/cluster/block/ClusterBlocks.java
+++ b/server/src/main/java/org/elasticsearch/cluster/block/ClusterBlocks.java
@@ -556,6 +556,8 @@ public void writeTo(StreamOutput out) throws IOException {
             out.writeMap(indices, (o, s) -> writeBlockSet(s, o));
             if (out.getTransportVersion().onOrAfter(TransportVersions.PROJECT_DELETION_GLOBAL_BLOCK)) {
                 writeBlockSet(projectGlobal, out);
+            } else {
+                assert projectGlobal.isEmpty() : "Any MP-enabled cluster must be past TransportVersions.PROJECT_DELETION_GLOBAL_BLOCK";
             }
         }
     }