diff --git a/Lib/fontTools/ttLib/tables/otBase.py b/Lib/fontTools/ttLib/tables/otBase.py
index f337f7075..0743eed0e 100644
--- a/Lib/fontTools/ttLib/tables/otBase.py
+++ b/Lib/fontTools/ttLib/tables/otBase.py
@@ -779,7 +779,7 @@ class FormatSwitchingBaseTable(BaseTable):
 		return NotImplemented
 
 	def getConverters(self):
-		return self.converters[self.Format]
+		return self.converters.get(self.Format, [])
 
 	def getConverterByName(self, name):
 		return self.convertersByName[self.Format][name]
diff --git a/Lib/fontTools/ttLib/tables/otTables.py b/Lib/fontTools/ttLib/tables/otTables.py
index 3bdb6f969..237864bb2 100644
--- a/Lib/fontTools/ttLib/tables/otTables.py
+++ b/Lib/fontTools/ttLib/tables/otTables.py
@@ -481,7 +481,8 @@ class Coverage(FormatSwitchingBaseTable):
 					endID = len(glyphOrder)
 				glyphs.extend(glyphOrder[glyphID] for glyphID in range(startID, endID))
 		else:
-			assert 0, "unknown format: %s" % self.Format
+			self.glyphs = []
+			log.warning("Unknown Coverage format: %s" % self.Format)
 
 	def preWrite(self, font):
 		glyphs = getattr(self, "glyphs", None)