diff --git a/Lib/fontTools/ttLib/tables/_c_m_a_p.py b/Lib/fontTools/ttLib/tables/_c_m_a_p.py
index dfb12ec06..57b81c474 100644
--- a/Lib/fontTools/ttLib/tables/_c_m_a_p.py
+++ b/Lib/fontTools/ttLib/tables/_c_m_a_p.py
@@ -322,9 +322,9 @@ class cmap_format_6(CmapSubtable):
 		firstCode = int(firstCode)
 		self.version = int(version)
 		data = data[10:]
-		assert len(data) == 2 * entryCount
+		#assert len(data) == 2 * entryCount  # XXX not true in Apple's Helvetica!!!
 		glyphIndexArray = array.array("H")
-		glyphIndexArray.fromstring(data)
+		glyphIndexArray.fromstring(data[:2 * entryCount])
 		if ttLib.endian <> "big":
 			glyphIndexArray.byteswap()
 		self.cmap = cmap = {}