diff --git a/Lib/fontTools/merge.py b/Lib/fontTools/merge.py
index 4009e14a1..36ac75659 100644
--- a/Lib/fontTools/merge.py
+++ b/Lib/fontTools/merge.py
@@ -380,10 +380,18 @@ def _glyphsAreSame(glyphSet1, glyphSet2, glyph1, glyph2):
 		g1.width == g2.width and
 		(not hasattr(g1, 'height') or g1.height == g2.height))
 
+# Valid (format, platformID, platEncID) triplets for cmap subtables containing
+# Unicode BMP-only and Unicode Full Repertoire semantics.
+# Cf. OpenType spec for "Platform specific encodings":
+# https://docs.microsoft.com/en-us/typography/opentype/spec/name
+class CmapUnicodePlatEncodings:
+	BMP = {(4, 3, 1), (4, 0, 3), (4, 0, 4), (4, 0, 6)}
+	FullRepertoire = {(12, 3, 10), (12, 0, 4), (12, 0, 6)}
+
 @_add_method(ttLib.getTableClass('cmap'))
 def merge(self, m, tables):
 	# TODO Handle format=14.
-	# Only merges 4/3/1 and 12/3/10 subtables, ignores all other subtables
+	# Only merge format 4 and 12 Unicode subtables, ignores all other subtables
 	# If there is a format 12 table for the same font, ignore the format 4 table
 	cmapTables = []
 	for fontIdx,table in enumerate(tables):
@@ -391,10 +399,16 @@ def merge(self, m, tables):
 		format12 = None
 		for subtable in table.tables:
 			properties = (subtable.format, subtable.platformID, subtable.platEncID)
-			if properties == (4,3,1):
+			if properties in CmapUnicodePlatEncodings.BMP:
 				format4 = subtable
-			elif properties == (12,3,10):
+			elif properties in CmapUnicodePlatEncodings.FullRepertoire:
 				format12 = subtable
+			else:
+				log.warning(
+					"Dropped cmap subtable from font [%s]:\t"
+					"format %2s, platformID %2s, platEncID %2s",
+					fontIdx, subtable.format, subtable.platformID, subtable.platEncID
+				)
 		if format12 is not None:
 			cmapTables.append((format12, fontIdx))
 		elif format4 is not None: