--- a/net/minecraft/server/DefinedStructureManager.java
+++ b/net/minecraft/server/DefinedStructureManager.java
@@ -20,7 +20,7 @@
 public class DefinedStructureManager implements IResourcePackListener {
 
     private static final Logger LOGGER = LogManager.getLogger();
-    private final Map<MinecraftKey, DefinedStructure> b = Maps.newHashMap();
+    private final Map<MinecraftKey, DefinedStructure> b = Maps.newConcurrentMap(); // SPIGOT-5287
     private final DataFixer c;
     private final MinecraftServer d;
     private final java.nio.file.Path e;
