diff --git a/zircon/system/host/banjo/lib/lexer.cpp b/zircon/system/host/banjo/lib/lexer.cpp
index 42cc9d2..859c6d5 100644
--- a/zircon/system/host/banjo/lib/lexer.cpp
+++ b/zircon/system/host/banjo/lib/lexer.cpp
@@ -7,6 +7,8 @@
 #include <assert.h>
 #include <ctype.h>
 
+#include <zircon/compiler.h>
+
 namespace banjo {
 
 namespace {
@@ -143,7 +145,7 @@
             // This escaping logic is incorrect for the input: "\\"
             if (last != '\\')
                 return Finish(Token::Kind::kStringLiteral);
-        // Fall through.
+        __FALLTHROUGH;
         default:
             last = next;
         }
@@ -224,7 +226,7 @@
             Consume();
             return Finish(Token::Kind::kArrow);
         }
-    // Fallthrough
+    __FALLTHROUGH;
     case '0':
     case '1':
     case '2':