diff --git a/Lib/fontTools/otlLib/builder.py b/Lib/fontTools/otlLib/builder.py
index f8b3ce3b0..1d5e7b6ee 100644
--- a/Lib/fontTools/otlLib/builder.py
+++ b/Lib/fontTools/otlLib/builder.py
@@ -479,7 +479,10 @@ def _getSinglePosValueKey(valueRecord):
     valueFormat, result = 0, []
     for name, value in valueRecord.__dict__.items():
         if isinstance(value, ot.Device):
-            result.append((name, _makeDeviceTuple(value)))
+            if value.DeltaFormat & 0x8000:
+                result.append((name, _makeVariationInstance(value)))
+            else:
+                result.append((name, _makeDeviceTuple(value)))
         else:
             result.append((name, value))
         valueFormat |= valueRecordFormatDict[name][0]
@@ -493,12 +496,20 @@ def _makeDeviceTuple(device):
     return (device.DeltaFormat, device.StartSize, device.EndSize,
             tuple(device.DeltaValue))
 
+def _makeVariationInstance(device):
+    """otTables.Device --> tuple, for making device tables unique"""
+    return (device.DeltaFormat, device.StartSize, device.EndSize)
+
 def _getSinglePosValueSize(valueKey):
     """Returns how many ushorts this valueKey (short form of ValueRecord) takes up"""
     count = 0
     for k in valueKey[1:]:
         if hasattr(k[1], '__len__') and len(k[1]):
-            count += len(k[1][3]) + 3
+            # it is a device table key
+            if k[1][0] & 0x8000:
+                count += 3
+            else:
+                count += len(k[1][3]) + 3
         else:
             count += 1
     return count