diff --git a/ClassicalSharp/Map/Formats/MapLvl.Importer.cs b/ClassicalSharp/Map/Formats/MapLvl.Importer.cs
index 26b5917ec..d629437a8 100644
--- a/ClassicalSharp/Map/Formats/MapLvl.Importer.cs
+++ b/ClassicalSharp/Map/Formats/MapLvl.Importer.cs
@@ -50,25 +50,42 @@ namespace ClassicalSharp.Map {
 			byte[] chunk = new byte[16 * 16 * 16];
 			byte[] data = new byte[1];
 			
+			// skip bounds checks when we know chunk is entirely inside map
+			int adjWidth  = width  & ~0x0F;
+			int adjHeight = height & ~0x0F;
+			int adjLength = length & ~0x0F;
+			
 			for (int y = 0; y < height; y += 16)
 				for (int z = 0; z < length; z += 16)
-					for (int x = 0; x < width; x += 16) 
+					for (int x = 0; x < width; x += 16)
 			{
 				int read = s.Read(data, 0, 1);
 				if (read == 0 || data[0] != 1) continue;
-				s.Read(chunk, 0, chunk.Length);
-				
+				s.Read(chunk, 0, chunk.Length);				
 				int baseIndex = (y * length + z) * width + x;
-				for (int i = 0; i < chunk.Length; i++) {
-					int xx = i & 0xF, yy = (i >> 8) & 0xF, zz = (i >> 4) & 0xF;
-					int index = baseIndex + (yy * length + zz) * width + xx;
-					
-					if (blocks[index] != customTile) continue;
-					blocks[index] = chunk[i];
+				
+				if ((x + 16) <= adjWidth && (y + 16) <= adjHeight && (z + 16) <= adjLength) {
+					for (int i = 0; i < chunk.Length; i++) {
+						int xx = i & 0xF, yy = (i >> 8) & 0xF, zz = (i >> 4) & 0xF;
+						int index = baseIndex + (yy * length + zz) * width + xx;
+						
+						if (blocks[index] != customTile) continue;
+						blocks[index] = chunk[i];
+					}
+				} else {
+					for (int i = 0; i < chunk.Length; i++) {
+						int xx = i & 0xF, yy = (i >> 8) & 0xF, zz = (i >> 4) & 0xF;
+						if ((x + xx) >= width || (y + yy) >= height || (z + zz) >= length) continue;
+						int index = baseIndex + (yy * length + zz) * width + xx;
+						
+						if (blocks[index] != customTile) continue;
+						blocks[index] = chunk[i];
+					}
 				}
+				
 			}
 		}
-			
+		
 		unsafe void ConvertPhysicsBlocks(byte[] blocks) {
 			byte* conv = stackalloc byte[256];
 			int count = Block.CpeCount;
diff --git a/src/Client/Formats.c b/src/Client/Formats.c
index f1fe6a85c..7dca3d20c 100644
--- a/src/Client/Formats.c
+++ b/src/Client/Formats.c
@@ -33,6 +33,11 @@ UInt8 Lvl_table[256 - BLOCK_CPE_COUNT] = { 0, 0, 0, 0, 39, 36, 36, 10, 46, 21, 2
 void Lvl_ReadCustomBlocks(Stream* stream) {
 	Int32 x, y, z, i;
 	UInt8 chunk[LVL_CHUNKSIZE * LVL_CHUNKSIZE * LVL_CHUNKSIZE];
+	/* skip bounds checks when we know chunk is entirely inside map */
+	Int32 adjWidth  = World_Width  & ~0x0F;
+	Int32 adjHeight = World_Height & ~0x0F;
+	Int32 adjLength = World_Length & ~0x0F;
+
 	for (y = 0; y < World_Height; y += LVL_CHUNKSIZE) {
 		for (z = 0; z < World_Length; z += LVL_CHUNKSIZE) {
 			for (x = 0; x < World_Width; x += LVL_CHUNKSIZE) {
@@ -40,11 +45,21 @@ void Lvl_ReadCustomBlocks(Stream* stream) {
 				Stream_Read(stream, chunk, sizeof(chunk));
 
 				Int32 baseIndex = World_Pack(x, y, z);
-				for (i = 0; i < sizeof(chunk); i++) {
-					Int32 xx = i & 0xF, yy = (i >> 8) & 0xF, zz = (i >> 4) & 0xF;
-					Int32 index = baseIndex + World_Pack(xx, yy, zz);
-					World_Blocks[index] = World_Blocks[index] == LVL_CUSTOMTILE ? chunk[i] : World_Blocks[index];
+				if ((x + LVL_CHUNKSIZE) <= adjWidth && (y + LVL_CHUNKSIZE) <= adjHeight && (z + LVL_CHUNKSIZE) <= adjLength) {
+					for (i = 0; i < sizeof(chunk); i++) {
+						Int32 xx = i & 0xF, yy = (i >> 8) & 0xF, zz = (i >> 4) & 0xF;
+						Int32 index = baseIndex + World_Pack(xx, yy, zz);
+						World_Blocks[index] = World_Blocks[index] == LVL_CUSTOMTILE ? chunk[i] : World_Blocks[index];
+					}
+				} else {
+					for (i = 0; i < sizeof(chunk); i++) {
+						Int32 xx = i & 0xF, yy = (i >> 8) & 0xF, zz = (i >> 4) & 0xF;
+						if ((x + xx) >= World_Width || (y + yy) >= World_Height || (z + zz) >= World_Length) continue;
+						Int32 index = baseIndex + World_Pack(xx, yy, zz);
+						World_Blocks[index] = World_Blocks[index] == LVL_CUSTOMTILE ? chunk[i] : World_Blocks[index];
+					}
 				}
+				
 			}
 		}
 	}