diff --git a/Lib/fontTools/ttLib/tables/_c_m_a_p.py b/Lib/fontTools/ttLib/tables/_c_m_a_p.py
index 3398a88..9dea010 100644
--- a/Lib/fontTools/ttLib/tables/_c_m_a_p.py
+++ b/Lib/fontTools/ttLib/tables/_c_m_a_p.py
@@ -712,7 +712,7 @@
 
 		if idDelta > 0x7FFF:
 			idDelta = idDelta - 0x10000
-		elif idDelta <  -0x7FFF:
+		elif idDelta <  -0x8000:
 			idDelta = idDelta + 0x10000
 
 		return idDelta