diff --git a/llvm/lib/Support/Compression.cpp b/llvm/lib/Support/Compression.cpp
index 497b17e..f22d3db 100644
--- a/llvm/lib/Support/Compression.cpp
+++ b/llvm/lib/Support/Compression.cpp
@@ -31,6 +31,7 @@
     case zlib::DefaultCompression: return Z_DEFAULT_COMPRESSION;
     case zlib::BestSizeCompression: return 9;
   }
+  llvm_unreachable("Invalid zlib::CompressionLevel!");
 }
 
 static zlib::Status encodeZlibReturnValue(int ReturnValue) {