diff --git a/Makefile b/Makefile
index 50dddc9760..d9ebbb72b1 100644
--- a/Makefile
+++ b/Makefile
@@ -539,7 +539,7 @@ endif
 
 ifdef USE_CLANG
 	# Used by some teams in Facebook
-	WARNING_FLAGS += -Wshift-sign-overflow -Wambiguous-reversed-operator
+	WARNING_FLAGS += -Wshift-sign-overflow -Wambiguous-reversed-operator -Wimplicit-fallthrough
 endif
 
 ifeq ($(PLATFORM), OS_OPENBSD)
diff --git a/cache/clock_cache.cc b/cache/clock_cache.cc
index fd330d90d8..e37c03fb50 100644
--- a/cache/clock_cache.cc
+++ b/cache/clock_cache.cc
@@ -51,14 +51,15 @@ inline uint64_t GetInitialCountdown(Cache::Priority priority) {
   switch (priority) {
     case Cache::Priority::HIGH:
       return ClockHandle::kHighCountdown;
-    default:
-      assert(false);
-      FALLTHROUGH_INTENDED;
     case Cache::Priority::LOW:
       return ClockHandle::kLowCountdown;
     case Cache::Priority::BOTTOM:
       return ClockHandle::kBottomCountdown;
   }
+  // Switch should have been exhaustive.
+  assert(false);
+  // For release build, fall back on something reasonable.
+  return ClockHandle::kLowCountdown;
 }
 
 inline void MarkEmpty(ClockHandle& h) {