diff --git a/lib/cmdlib.py b/lib/cmdlib.py
index e472dac00f431ff2cfbddeb4725e64fb35ba53f2..3b099942933e11b70d986fe64b5006951889e3d3 100644
--- a/lib/cmdlib.py
+++ b/lib/cmdlib.py
@@ -13288,7 +13288,7 @@ class LUInstanceSetParams(LogicalUnit):
                                  errors.ECODE_STATE)
     disk_sizes = [disk.size for disk in instance.disks]
     disk_sizes.extend(params["size"] for (op, idx, params, private) in
-                      self.diskmod)
+                      self.diskmod if op == constants.DDM_ADD)
     ispec[constants.ISPEC_DISK_COUNT] = len(disk_sizes)
     ispec[constants.ISPEC_DISK_SIZE] = disk_sizes