diff --git a/Lib/fontTools/ttLib/tables/L_T_S_H_.py b/Lib/fontTools/ttLib/tables/L_T_S_H_.py
index 20701d61b..62c4b9f83 100644
--- a/Lib/fontTools/ttLib/tables/L_T_S_H_.py
+++ b/Lib/fontTools/ttLib/tables/L_T_S_H_.py
@@ -12,8 +12,8 @@ class table_L_T_S_H_(DefaultTable.DefaultTable):
 	def decompile(self, data, ttFont):
 		version, numGlyphs = struct.unpack(">HH", data[:4])
 		data = data[4:]
-		assert version == 0
-		assert len(data) == numGlyphs
+		assert version == 0, "unknown version: %s" % version
+		assert len(data) == numGlyphs, "numGlyphs doesn't match data length"
 		# ouch: the assertion is not true in Chicago!
 		#assert numGlyphs == ttFont['maxp'].numGlyphs
 		yPels = array.array("B")