diff --git a/include/llvm/CodeGen/SelectionDAGNodes.h b/include/llvm/CodeGen/SelectionDAGNodes.h
index afd1e78d204..025e9ecea7c 100644
--- a/include/llvm/CodeGen/SelectionDAGNodes.h
+++ b/include/llvm/CodeGen/SelectionDAGNodes.h
@@ -529,6 +529,7 @@ namespace ISD {
     // a call sequence, and carry arbitrary information that target might want
     // to know.  The first operand is a chain, the rest are specified by the
     // target and not touched by the DAG optimizers.
+    // CALLSEQ_START..CALLSEQ_END pairs may not be nested.
     CALLSEQ_START,  // Beginning of a call sequence
     CALLSEQ_END,    // End of a call sequence
     
diff --git a/lib/CodeGen/SelectionDAG/LegalizeDAG.cpp b/lib/CodeGen/SelectionDAG/LegalizeDAG.cpp
index 82324cd5b99..fd9cf154436 100644
--- a/lib/CodeGen/SelectionDAG/LegalizeDAG.cpp
+++ b/lib/CodeGen/SelectionDAG/LegalizeDAG.cpp
@@ -1266,6 +1266,13 @@ SDOperand SelectionDAGLegalize::LegalizeOp(SDOperand Op) {
     // The only option for this is to custom lower it.
     Tmp3 = TLI.LowerOperation(Result.getValue(0), DAG);
     assert(Tmp3.Val && "Target didn't custom lower this node!");
+    // A call within a calling sequence must be legalized to something
+    // other than the normal CALLSEQ_END.  Violating this gets Legalize
+    // into an infinite loop.
+    assert ((!IsLegalizingCall ||
+             Node->getOpcode() != ISD::CALL ||
+             Tmp3.Val->getOpcode() != ISD::CALLSEQ_END) &&
+            "Nested CALLSEQ_START..CALLSEQ_END not supported.");
 
     // The number of incoming and outgoing values should match; unless the final
     // outgoing value is a flag.