diff --git a/src/Simulator/VanillaFluidSimulator.cpp b/src/Simulator/VanillaFluidSimulator.cpp
index 18d9b07e1..6df75eebb 100644
--- a/src/Simulator/VanillaFluidSimulator.cpp
+++ b/src/Simulator/VanillaFluidSimulator.cpp
@@ -98,7 +98,7 @@ int cVanillaFluidSimulator::CalculateFlowCost(cChunk * a_Chunk, int a_RelX, int
 	}
 
 	// Check if block below is passable
-	if (!a_Chunk->UnboundedRelGetBlock(a_RelX, a_RelY - 1, a_RelZ, BlockType, BlockMeta))
+	if ((a_RelY > 0) && !a_Chunk->UnboundedRelGetBlock(a_RelX, a_RelY - 1, a_RelZ, BlockType, BlockMeta))
 	{
 		return Cost;
 	}