diff --git a/core/src/main/java/org/elasticsearch/index/shard/ShardPath.java b/core/src/main/java/org/elasticsearch/index/shard/ShardPath.java
index be0d51bd2b6..1b160d5fad5 100644
--- a/core/src/main/java/org/elasticsearch/index/shard/ShardPath.java
+++ b/core/src/main/java/org/elasticsearch/index/shard/ShardPath.java
@@ -173,7 +173,7 @@ public final class ShardPath {
         } else {
             long totFreeSpace = 0;
             for (NodeEnvironment.NodePath nodePath : env.nodePaths()) {
-                totFreeSpace += nodePath.fileStore.getUsableSpace();
+                totFreeSpace += Math.max(0, nodePath.fileStore.getUsableSpace());
             }
 
             // TODO: this is a hack!!  We should instead keep track of incoming (relocated) shards since we know
@@ -189,12 +189,15 @@ public final class ShardPath {
             long maxUsableBytes = Long.MIN_VALUE;
             for (NodeEnvironment.NodePath nodePath : paths) {
                 FileStore fileStore = nodePath.fileStore;
-                long usableBytes = fileStore.getUsableSpace();
+
+                // Apparently, FileStore.getUsableSpace() can sometimes be negative, even possibly Long.MIN_VALUE, which can lead to NPE
+                // below since we would fail to set bestPath:
+                long usableBytes = Math.max(0, fileStore.getUsableSpace());
 
                 // Deduct estimated reserved bytes from usable space:
                 Integer count = dataPathToShardCount.get(nodePath.path);
                 if (count != null) {
-                    usableBytes -= estShardSizeInBytes * count;
+                    usableBytes = Math.subtractExact(usableBytes, Math.multiplyExact(estShardSizeInBytes, count));
                 }
                 if (usableBytes > maxUsableBytes) {
                     maxUsableBytes = usableBytes;
diff --git a/core/src/test/java/org/elasticsearch/index/shard/NewPathForShardTests.java b/core/src/test/java/org/elasticsearch/index/shard/NewPathForShardTests.java
index f1515cd559b..2698de2d840 100644
--- a/core/src/test/java/org/elasticsearch/index/shard/NewPathForShardTests.java
+++ b/core/src/test/java/org/elasticsearch/index/shard/NewPathForShardTests.java
@@ -211,4 +211,31 @@ public class NewPathForShardTests extends ESTestCase {
 
         nodeEnv.close();
     }
+
+    public void testNegativeUsableSpace() throws Exception {
+        Path path = PathUtils.get(createTempDir().toString());
+
+        // Use 1 data path:
+        String[] paths = new String[] {path.resolve("a").toString()};
+
+        Settings settings = Settings.builder()
+            .put(Environment.PATH_HOME_SETTING.getKey(), path)
+            .putArray(Environment.PATH_DATA_SETTING.getKey(), paths).build();
+        NodeEnvironment nodeEnv = new NodeEnvironment(settings, new Environment(settings));
+
+        // Make sure all our mocking above actually worked:
+        NodePath[] nodePaths = nodeEnv.nodePaths();
+        assertEquals(1, nodePaths.length);
+
+        assertEquals("mocka", nodePaths[0].fileStore.name());
+
+        // Path a is confused about its free space:
+        aFileStore.usableSpace = -Long.MIN_VALUE;
+
+        ShardId shardId = new ShardId("index", "_na_", 0);
+        ShardPath result = ShardPath.selectNewPathForShard(nodeEnv, shardId, INDEX_SETTINGS, 100, Collections.<Path,Integer>emptyMap());
+        assertTrue(result.getDataPath().toString().contains(aPathPart));
+
+        nodeEnv.close();
+    }
 }