diff --git a/lib/cmdlib.py b/lib/cmdlib.py
index 421f07169d0844030b501c28053a46155e80d7c7..2e641d84578b39d308d7b6542bc8c390d2c41f4d 100644
--- a/lib/cmdlib.py
+++ b/lib/cmdlib.py
@@ -10148,6 +10148,14 @@ class LUNodeEvacuate(NoHooksLU):
     else:
       # All instances
       assert self.op.mode == constants.IALLOCATOR_NEVAC_ALL
+      # TODO: In 2.6, change the iallocator interface to take an evacuation mode
+      # per instance
+      raise errors.OpPrereqError("Due to an issue with the iallocator"
+                                 " interface it is not possible to evacuate"
+                                 " all instances at once; specify explicitly"
+                                 " whether to evacuate primary or secondary"
+                                 " instances",
+                                 errors.ECODE_INVAL)
       inst_fn = _GetNodeInstances
 
     return inst_fn(self.cfg, self.op.node_name)