WARNING Task #5121 for RailDriver v0.5 generated an exceptio

15.05 10:09:09 [Server] INFO at net.minecraft.server.v1_5_R3.ThreadServerApplication.run(SourceFile:573)
15.05 10:09:09 [Server] INFO at net.minecraft.server.v1_5_R3.MinecraftServer.run(MinecraftServer.java:410)
15.05 10:09:09 [Server] INFO at net.minecraft.server.v1_5_R3.MinecraftServer.q(MinecraftServer.java:477)
15.05 10:09:09 [Server] INFO at net.minecraft.server.v1_5_R3.DedicatedServer.r(DedicatedServer.java:226)
15.05 10:09:09 [Server] INFO at net.minecraft.server.v1_5_R3.MinecraftServer.r(MinecraftServer.java:513)
15.05 10:09:09 [Server] INFO at org.bukkit.craftbukkit.v1_5_R3.scheduler.CraftScheduler.mainThreadHeartbeat(CraftScheduler.java:345)
15.05 10:09:09 [Server] INFO at org.bukkit.craftbukkit.v1_5_R3.scheduler.CraftTask.run(CraftTask.java:53)
15.05 10:09:09 [Server] INFO at ca.mcpnet.RailDriver.RailDriverTask.run(RailDriverTask.java:161)
15.05 10:09:09 [Server] INFO at ca.mcpnet.RailDriver.RailDriverTask.ejectItems(RailDriverTask.java:535)
15.05 10:09:09 [Server] INFO at org.bukkit.craftbukkit.v1_5_R3.CraftWorld.dropItem(CraftWorld.java:296)
15.05 10:09:09 [Server] INFO at org.apache.commons.lang.Validate.isTrue(Validate.java:157)
15.05 10:09:09 [Server] INFO java.lang.IllegalArgumentException: Cannot drop AIR.
15.05 10:09:09 [Server] WARNING Task #5121 for RailDriver v0.5 generated an exception