diff --git a/include/llvm/Target/TargetLowering.h b/include/llvm/Target/TargetLowering.h
index dd23d8cac56e..29ecedcc6432 100644
--- a/include/llvm/Target/TargetLowering.h
+++ b/include/llvm/Target/TargetLowering.h
@@ -2030,6 +2030,7 @@ class TargetLowering : public TargetLoweringBase {
                            ISD::CondCode &CCCode, SDLoc DL) const;
 
   /// Returns a pair of (return value, chain).
+  /// It is an error to pass RTLIB::UNKNOWN_LIBCALL as \p LC.
   std::pair<SDValue, SDValue> makeLibCall(SelectionDAG &DAG, RTLIB::Libcall LC,
                                           EVT RetVT, const SDValue *Ops,
                                           unsigned NumOps, bool isSigned,
diff --git a/lib/CodeGen/SelectionDAG/TargetLowering.cpp b/lib/CodeGen/SelectionDAG/TargetLowering.cpp
index c382fe0d6fb9..ddbf0b2dd532 100644
--- a/lib/CodeGen/SelectionDAG/TargetLowering.cpp
+++ b/lib/CodeGen/SelectionDAG/TargetLowering.cpp
@@ -100,6 +100,8 @@ TargetLowering::makeLibCall(SelectionDAG &DAG,
     Entry.isZExt = !shouldSignExtendTypeInLibCall(Ops[i].getValueType(), isSigned);
     Args.push_back(Entry);
   }
+  if (LC == RTLIB::UNKNOWN_LIBCALL)
+    report_fatal_error("Unsupported library call operation!");
   SDValue Callee = DAG.getExternalSymbol(getLibcallName(LC), getPointerTy());
 
   Type *RetTy = RetVT.getTypeForEVT(*DAG.getContext());