diff --git a/lib/Target/Hexagon/HexagonSubtarget.cpp b/lib/Target/Hexagon/HexagonSubtarget.cpp
index ce81a78..5d087db 100644
--- a/lib/Target/Hexagon/HexagonSubtarget.cpp
+++ b/lib/Target/Hexagon/HexagonSubtarget.cpp
@@ -55,7 +55,11 @@
   case HexagonSubtarget::V5:
     break;
   default:
-    llvm_unreachable("Unknown Architecture Version.");
+    // If the programmer has not specified a Hexagon version, default
+    // to -mv4.
+    CPUString = "hexagonv4";
+    HexagonArchVersion = HexagonSubtarget::V4;
+    break;
   }
 
   // Initialize scheduling itinerary for the specified CPU.