diff --git a/include/llvm/Analysis/Dominators.h b/include/llvm/Analysis/Dominators.h
index b405f5b71ed..347e239d8ea 100644
--- a/include/llvm/Analysis/Dominators.h
+++ b/include/llvm/Analysis/Dominators.h
@@ -270,12 +270,17 @@ protected:
         NewBBIDom = PredBlocks[i];
         break;
       }
-    assert(i != PredBlocks.size() && "No reachable preds?");
+
+    // It's possible that none of the predecessors of NewBB are reachable;
+    // in that case, NewBB itself is unreachable, so nothing needs to be
+    // changed.
+    if (!NewBBIDom)
+      return;
+
     for (i = i + 1; i < PredBlocks.size(); ++i) {
       if (DT.isReachableFromEntry(PredBlocks[i]))
         NewBBIDom = DT.findNearestCommonDominator(NewBBIDom, PredBlocks[i]);
     }
-    assert(NewBBIDom && "No immediate dominator found??");
 
     // Create the new dominator tree node... and set the idom of NewBB.
     DomTreeNodeBase<NodeT> *NewBBNode = DT.addNewBlock(NewBB, NewBBIDom);
diff --git a/test/CodeGen/Generic/2009-06-03-UnreachableSplitPad.ll b/test/CodeGen/Generic/2009-06-03-UnreachableSplitPad.ll
new file mode 100644
index 00000000000..59e7d0c7a8f
--- /dev/null
+++ b/test/CodeGen/Generic/2009-06-03-UnreachableSplitPad.ll
@@ -0,0 +1,15 @@
+; RUN: llvm-as < %s | llc
+; PR4317
+
+declare i32 @b()
+
+define void @a() {
+entry:
+  ret void
+
+dummy:
+  invoke i32 @b() to label %reg unwind label %reg
+
+reg:
+  ret void
+}