diff --git a/utils/TableGen/CodeGenInstruction.cpp b/utils/TableGen/CodeGenInstruction.cpp
index 0ee6b41..8b70647 100644
--- a/utils/TableGen/CodeGenInstruction.cpp
+++ b/utils/TableGen/CodeGenInstruction.cpp
@@ -163,7 +163,7 @@
       isVariadic = true;
       continue;
     } else if (!Rec->isSubClassOf("RegisterClass") && 
-               Rec->getName() != "ptr_rc")
+               Rec->getName() != "ptr_rc" && Rec->getName() != "unknown")
       throw "Unknown operand class '" + Rec->getName() +
             "' in instruction '" + R->getName() + "' instruction!";