diff --git a/meson.build b/meson.build
index bede78a64..18bdc98b9 100644
--- a/meson.build
+++ b/meson.build
@@ -586,6 +586,8 @@ add_project_arguments(
   '-Wdeprecated-copy',
   '-Wignored-qualifiers',
   '-Werror=suggest-override',
+  # Undefined behavior.
+  '-Werror=embedded-directive',
   # don't let nlohmann automatically cast enums to ints. we don't set this
   # inside libutil headers to not force this behavior on users of libutil.
   '-DJSON_DISABLE_ENUM_SERIALIZATION=1',
diff --git a/tests/unit/libstore/serve-protocol.cc b/tests/unit/libstore/serve-protocol.cc
index c3638f898..c85767196 100644
--- a/tests/unit/libstore/serve-protocol.cc
+++ b/tests/unit/libstore/serve-protocol.cc
@@ -212,13 +212,11 @@ VERSIONED_CHARACTERIZATION_TEST(
                 },
                 .startTime = 30,
                 .stopTime = 50,
-#if 0
                 // These fields are not yet serialized.
                 // FIXME Include in next version of protocol or document
                 // why they are skipped.
-                .cpuUser = std::chrono::milliseconds(500s),
-                .cpuSystem = std::chrono::milliseconds(604s),
-#endif
+                // .cpuUser = std::chrono::milliseconds(500s),
+                // .cpuSystem = std::chrono::milliseconds(604s),
             },
         };
         t;