diff --git a/runtime/monitor.cc b/runtime/monitor.cc
index 6479283..7240357 100644
--- a/runtime/monitor.cc
+++ b/runtime/monitor.cc
@@ -168,11 +168,11 @@
     }
     case LockWord::kUnlocked: {
       LOG(FATAL) << "Inflating unlocked lock word";
-      break;
+      UNREACHABLE();
     }
     default: {
       LOG(FATAL) << "Invalid monitor state " << lw.GetState();
-      return false;
+      UNREACHABLE();
     }
   }
   LockWord fat(this, lw.GCState());
@@ -1236,7 +1236,7 @@
       }
       default: {
         LOG(FATAL) << "Invalid monitor state " << lock_word.GetState();
-        return false;
+        UNREACHABLE();
       }
     }
   }
@@ -1280,7 +1280,7 @@
       case LockWord::kFatLocked:  // Unreachable given the loop condition above. Fall-through.
       default: {
         LOG(FATAL) << "Invalid monitor state " << lock_word.GetState();
-        return;
+        UNREACHABLE();
       }
     }
   }
@@ -1320,7 +1320,7 @@
     }
     default: {
       LOG(FATAL) << "Invalid monitor state " << lock_word.GetState();
-      return;
+      UNREACHABLE();
     }
   }
 }