Index: lib/CodeGen/MachineDominators.cpp
===================================================================
--- lib/CodeGen/MachineDominators.cpp
+++ lib/CodeGen/MachineDominators.cpp
@@ -143,6 +143,11 @@
 }
 
 void MachineDominatorTree::verifyDomTree() const {
+  if (getRoots().empty())
+    // If no roots found in the dominator tree, the pass that builds it is
+    // likely to be skipped.
+    return;
+
   MachineFunction &F = *getRoot()->getParent();
 
   MachineDominatorTree OtherDT;
Index: test/CodeGen/Generic/externally_available.ll
===================================================================
--- test/CodeGen/Generic/externally_available.ll
+++ test/CodeGen/Generic/externally_available.ll
@@ -1,4 +1,4 @@
-; RUN: llc < %s | not grep test_
+; RUN: llc -verify-machine-dom-info < %s | not grep test_
 
 ; test_function should not be emitted to the .s file.
 define available_externally i32 @test_function() {