diff --git a/build/flags.mk b/build/flags.mk
index c50b077c..e0d67d54 100644
--- a/build/flags.mk
+++ b/build/flags.mk
@@ -90,6 +90,7 @@ CFLAGS ?= \
     -Werror=implicit \
     -Wimplicit-fallthrough \
     -Wmissing-declarations \
+    -Wmissing-variable-declarations \
     -Wshadow \
     -Wsign-compare \
     -Wstrict-prototypes
diff --git a/tests/trie.c b/tests/trie.c
index 46673225..ebaae5d7 100644
--- a/tests/trie.c
+++ b/tests/trie.c
@@ -8,7 +8,7 @@
 #include <stdlib.h>
 #include <string.h>
 
-const char *keys[] = {
+static const char *keys[] = {
 	"foo",
 	"bar",
 	"baz",
@@ -37,7 +37,7 @@ const char *keys[] = {
 	">>>",
 };
 
-const size_t nkeys = countof(keys);
+static const size_t nkeys = countof(keys);
 
 bool check_trie(void) {
 	bool ret = true;