diff --git a/Lib/fontTools/ttLib/tables/otTables.py b/Lib/fontTools/ttLib/tables/otTables.py
index fa00ca054..302c9b2fe 100644
--- a/Lib/fontTools/ttLib/tables/otTables.py
+++ b/Lib/fontTools/ttLib/tables/otTables.py
@@ -1415,6 +1415,7 @@ def splitMarkBasePos(oldSubTable, newSubTable, overflowRecord):
 			oldMarkCoverage.append(glyphName)
 			oldMarkRecords.append(markRecord)
 		else:
+			markRecord.Class -= oldClassCount
 			newMarkCoverage.append(glyphName)
 			newMarkRecords.append(markRecord)