diff --git a/OpenRA.Mods.Common/UtilityCommands/ImportLegacyMapCommand.cs b/OpenRA.Mods.Common/UtilityCommands/ImportLegacyMapCommand.cs
index 762339c121..8e0cd6112a 100644
--- a/OpenRA.Mods.Common/UtilityCommands/ImportLegacyMapCommand.cs
+++ b/OpenRA.Mods.Common/UtilityCommands/ImportLegacyMapCommand.cs
@@ -62,6 +62,9 @@ namespace OpenRA.Mods.Common.UtilityCommands
 
 				// The original game isn't case sensitive, but we are.
 				var tileset = GetTileset(mapSection).ToUpperInvariant();
+				if (!modData.DefaultTileSets.ContainsKey(tileset))
+					throw new InvalidDataException("Unknown tileset {0}".F(tileset));
+
 				Map = new Map(modData, modData.DefaultTileSets[tileset], MapSize, MapSize)
 				{
 					Title = basic.GetValue("Name", Path.GetFileNameWithoutExtension(filename)),