Index: llvm/trunk/lib/TableGen/TGParser.cpp
===================================================================
--- llvm/trunk/lib/TableGen/TGParser.cpp
+++ llvm/trunk/lib/TableGen/TGParser.cpp
@@ -536,8 +536,14 @@
   }
 
   Record *Result = Records.getClass(Lex.getCurStrVal());
-  if (!Result)
-    TokError("Couldn't find class '" + Lex.getCurStrVal() + "'");
+  if (!Result) {
+    std::string Msg("Couldn't find class '" + Lex.getCurStrVal() + "'");
+    if (MultiClasses[Lex.getCurStrVal()].get())
+      TokError(Msg + ". Use 'defm' if you meant to use multiclass '" +
+               Lex.getCurStrVal() + "'");
+    else
+      TokError(Msg);
+  }
 
   Lex.Lex();
   return Result;
Index: llvm/trunk/test/TableGen/MultiClass-def-fail.td
===================================================================
--- llvm/trunk/test/TableGen/MultiClass-def-fail.td
+++ llvm/trunk/test/TableGen/MultiClass-def-fail.td
@@ -0,0 +1,10 @@
+// RUN: not llvm-tblgen %s 2>&1 | FileCheck %s
+// XFAIL: vg_leak
+
+// This test checks that using def instead of defm gives a meaningful error
+multiclass M2 {
+  def X;
+}
+
+// CHECK: error: Couldn't find class 'M2'. Use 'defm' if you meant to use multiclass 'M2'
+def rec1 : M2;