diff --git a/src/mapgen/mapgen_v6.cpp b/src/mapgen/mapgen_v6.cpp
index 2b4d37c6bd..a56ce24ea0 100644
--- a/src/mapgen/mapgen_v6.cpp
+++ b/src/mapgen/mapgen_v6.cpp
@@ -43,6 +43,18 @@ const FlagDesc flagdesc_mapgen_v6[] = {
 MapgenV6::MapgenV6(MapgenV6Params *params, EmergeParams *emerge)
 	: Mapgen(MAPGEN_V6, params, emerge)
 {
+	if (csize.X != csize.Y || csize.Y != csize.Z) {
+		// In practice:
+		// (5,2,1) segfaults in mudflow
+		// (5,2,5) generates very broken terrain
+		throw BaseException("MapgenV6: chunk size must be cubic");
+	}
+	if (csize.Y % 2 == 0) {
+		// weird ledges appear in some places
+		warningstream << "MapgenV6: chunk heights divisible by two are known "
+			"to be buggy." << std::endl;
+	}
+
 	ystride = csize.X;
 
 	heightmap = new s16[csize.X * csize.Z];
diff --git a/src/mapgen/mg_decoration.cpp b/src/mapgen/mg_decoration.cpp
index 1e3b7ec53d..600de1446d 100644
--- a/src/mapgen/mg_decoration.cpp
+++ b/src/mapgen/mg_decoration.cpp
@@ -132,6 +132,10 @@ void Decoration::placeDeco(Mapgen *mg, u32 blockseed, v3s16 nmin, v3s16 nmax)
 
 	PcgRandom ps(blockseed + 53);
 	int carea_size = nmax.X - nmin.X + 1;
+	if (nmax.Z - nmin.Z + 1 != carea_size) {
+		// TODO: this is a stupid restriction, which we should lift
+		throw BaseException("Decoration::placeDeco requires a square area (XZ)");
+	}
 
 	// Divide area into parts
 	// If chunksize is changed it may no longer be divisable by sidelen