diff --git a/server/src/main/java/org/elasticsearch/cluster/routing/allocation/decider/DiskThresholdDecider.java b/server/src/main/java/org/elasticsearch/cluster/routing/allocation/decider/DiskThresholdDecider.java
index 2a323af5f8435..ad30dc49a5524 100644
--- a/server/src/main/java/org/elasticsearch/cluster/routing/allocation/decider/DiskThresholdDecider.java
+++ b/server/src/main/java/org/elasticsearch/cluster/routing/allocation/decider/DiskThresholdDecider.java
@@ -409,11 +409,14 @@ public static long getExpectedShardSize(ShardRouting shard, RoutingAllocation al
             // the worst case
             long targetShardSize = 0;
             final Index mergeSourceIndex = metaData.getResizeSourceIndex();
-            final IndexMetaData sourceIndexMeta = allocation.metaData().getIndexSafe(mergeSourceIndex);
-            final Set<ShardId> shardIds = IndexMetaData.selectRecoverFromShards(shard.id(), sourceIndexMeta, metaData.getNumberOfShards());
-            for (IndexShardRoutingTable shardRoutingTable : allocation.routingTable().index(mergeSourceIndex.getName())) {
-                if (shardIds.contains(shardRoutingTable.shardId())) {
-                    targetShardSize += info.getShardSize(shardRoutingTable.primaryShard(), 0);
+            final IndexMetaData sourceIndexMeta = allocation.metaData().index(mergeSourceIndex);
+            if (sourceIndexMeta != null) {
+                final Set<ShardId> shardIds = IndexMetaData.selectRecoverFromShards(shard.id(),
+                    sourceIndexMeta, metaData.getNumberOfShards());
+                for (IndexShardRoutingTable shardRoutingTable : allocation.routingTable().index(mergeSourceIndex.getName())) {
+                    if (shardIds.contains(shardRoutingTable.shardId())) {
+                        targetShardSize += info.getShardSize(shardRoutingTable.primaryShard(), 0);
+                    }
                 }
             }
             return targetShardSize == 0 ? defaultValue : targetShardSize;
diff --git a/server/src/test/java/org/elasticsearch/cluster/routing/allocation/decider/DiskThresholdDeciderUnitTests.java b/server/src/test/java/org/elasticsearch/cluster/routing/allocation/decider/DiskThresholdDeciderUnitTests.java
index 3676ca8bd6e85..10fc358e4d4ea 100644
--- a/server/src/test/java/org/elasticsearch/cluster/routing/allocation/decider/DiskThresholdDeciderUnitTests.java
+++ b/server/src/test/java/org/elasticsearch/cluster/routing/allocation/decider/DiskThresholdDeciderUnitTests.java
@@ -342,6 +342,20 @@ public void testSizeShrinkIndex() {
         target2 = ShardRouting.newUnassigned(new ShardId(new Index("target2", "9101112"), 1),
             true, LocalShardsRecoverySource.INSTANCE, new UnassignedInfo(UnassignedInfo.Reason.INDEX_CREATED, "foo"));
         assertEquals(1000L, DiskThresholdDecider.getExpectedShardSize(target2, allocation, 0));
+
+        // check that the DiskThresholdDecider still works even if the source index has been deleted
+        ClusterState clusterStateWithMissingSourceIndex = ClusterState.builder(clusterState)
+            .metaData(MetaData.builder(metaData).remove("test"))
+            .routingTable(RoutingTable.builder(clusterState.routingTable()).remove("test").build())
+            .build();
+
+        allocationService.reroute(clusterState, "foo");
+
+        RoutingAllocation allocationWithMissingSourceIndex = new RoutingAllocation(null,
+            clusterStateWithMissingSourceIndex.getRoutingNodes(), clusterStateWithMissingSourceIndex, info, 0);
+
+        assertEquals(42L, DiskThresholdDecider.getExpectedShardSize(target, allocationWithMissingSourceIndex, 42L));
+        assertEquals(42L, DiskThresholdDecider.getExpectedShardSize(target2, allocationWithMissingSourceIndex, 42L));
     }
 
 }