diff --git a/llvm/lib/TableGen/Record.cpp b/llvm/lib/TableGen/Record.cpp
--- a/llvm/lib/TableGen/Record.cpp
+++ b/llvm/lib/TableGen/Record.cpp
@@ -887,13 +887,18 @@
     if (LHSs && RHSs) {
       DefInit *LOp = dyn_cast<DefInit>(LHSs->getOperator());
       DefInit *ROp = dyn_cast<DefInit>(RHSs->getOperator());
-      if (!LOp || !ROp)
+      if ((!LOp && !isa<UnsetInit>(LHSs->getOperator())) ||
+          (!ROp && !isa<UnsetInit>(RHSs->getOperator())))
         break;
-      if (LOp->getDef() != ROp->getDef()) {
+      if (LOp && ROp && LOp->getDef() != ROp->getDef()) {
         PrintFatalError(Twine("Concatenated Dag operators do not match: '") +
                         LHSs->getAsString() + "' vs. '" + RHSs->getAsString() +
                         "'");
       }
+      Init *Op = LOp ? LOp : ROp;
+      if (!Op)
+        Op = UnsetInit::get();
+
       SmallVector<Init*, 8> Args;
       SmallVector<StringInit*, 8> ArgNames;
       for (unsigned i = 0, e = LHSs->getNumArgs(); i != e; ++i) {
@@ -904,7 +909,7 @@
         Args.push_back(RHSs->getArg(i));
         ArgNames.push_back(RHSs->getArgName(i));
       }
-      return DagInit::get(LHSs->getOperator(), nullptr, Args, ArgNames);
+      return DagInit::get(Op, nullptr, Args, ArgNames);
     }
     break;
   }
diff --git a/llvm/lib/TableGen/TGParser.cpp b/llvm/lib/TableGen/TGParser.cpp
--- a/llvm/lib/TableGen/TGParser.cpp
+++ b/llvm/lib/TableGen/TGParser.cpp
@@ -2024,7 +2024,8 @@
   }
   case tgtok::l_paren: {         // Value ::= '(' IDValue DagArgList ')'
     Lex.Lex();   // eat the '('
-    if (Lex.getCode() != tgtok::Id && Lex.getCode() != tgtok::XCast) {
+    if (Lex.getCode() != tgtok::Id && Lex.getCode() != tgtok::XCast &&
+        Lex.getCode() != tgtok::question) {
       TokError("expected identifier in dag init");
       return nullptr;
     }
diff --git a/llvm/test/TableGen/unsetop.td b/llvm/test/TableGen/unsetop.td
new file mode 100644
--- /dev/null
+++ b/llvm/test/TableGen/unsetop.td
@@ -0,0 +1,24 @@
+// RUN: llvm-tblgen %s | FileCheck %s
+// RUN: not llvm-tblgen -DERROR %s 2>&1 | FileCheck --check-prefix=ERROR %s
+
+def op;
+def otherop;
+
+def test {
+  // CHECK: dag d = (? "hello":$world);
+  dag d = (? "hello":$world);
+
+  // CHECK: dag undefNeither = (op 1, 2);
+  dag undefNeither = !con((op 1), (op 2));
+  // CHECK: dag undefFirst = (op 1, 2);
+  dag undefFirst   = !con((?  1), (op 2));
+  // CHECK: dag undefSecond = (op 1, 2);
+  dag undefSecond  = !con((op 1), (?  2));
+  // CHECK: dag undefBoth = (? 1, 2);
+  dag undefBoth    = !con((?  1), (?  2));
+
+#ifdef ERROR
+  // ERROR: Concatenated Dag operators do not match: '(op 1)' vs. '(otherop 2)'
+  dag mismatch     = !con((op 1), (otherop 2));
+#endif
+}