--- ../src-base/minecraft/net/minecraft/world/gen/structure/StructureStart.java
+++ ../src-work/minecraft/net/minecraft/world/gen/structure/StructureStart.java
@@ -63,6 +63,10 @@
 
     public NBTTagCompound func_143021_a(int p_143021_1_, int p_143021_2_)
     {
+        if (MapGenStructureIO.func_143033_a(this) == null) // This is just a more friendly error instead of the 'Null String' below
+        {
+            throw new RuntimeException("StructureStart \"" + this.getClass().getName() + "\" missing ID Mapping, Modder see MapGenStructureIO");
+        }
         NBTTagCompound nbttagcompound = new NBTTagCompound();
         nbttagcompound.func_74778_a("id", MapGenStructureIO.func_143033_a(this));
         nbttagcompound.func_74768_a("ChunkX", p_143021_1_);
