diff --git a/pocket-reform-keyboard-fw/pocket-hid/build.sh b/pocket-reform-keyboard-fw/pocket-hid/build.sh
index 907e5ae..4c0836c 100755
--- a/pocket-reform-keyboard-fw/pocket-hid/build.sh
+++ b/pocket-reform-keyboard-fw/pocket-hid/build.sh
@@ -1,8 +1,14 @@
 #!/bin/bash
 
+set -e
+
+mkdir -p build
+
+cc test/matrix-valid.c -o build/matrix-valid
+build/matrix-valid
+
 export PICO_SDK_PATH=$(pwd)/../../pico-sdk
 
-mkdir -p build
 cd build
 cmake -DFAMILY=rp2040 ..
 
diff --git a/pocket-reform-keyboard-fw/pocket-hid/test/matrix-valid.c b/pocket-reform-keyboard-fw/pocket-hid/test/matrix-valid.c
new file mode 100644
index 0000000..60e107a
--- /dev/null
+++ b/pocket-reform-keyboard-fw/pocket-hid/test/matrix-valid.c
@@ -0,0 +1,86 @@
+#define KBD_COLS 12
+#define KBD_ROWS 6
+#define KBD_MATRIX_SZ KBD_COLS * KBD_ROWS + 4
+
+#include <stdio.h>
+#include <stdint.h>
+#include "../src/matrix.h"
+
+#define WHY_HYPER_RETURN \
+  "Hyper+Return is required both to power on and to enter the keyboard menu.\n"
+
+#define WHY_X \
+  "Pressing X in the keyboard menu is required to enter bootloader mode.\n" \
+  "(Bootloader mode can be exited by toggling the standby power switch.)\n"
+
+int main() {
+  int i;
+  int hyper_loc = -1;
+  int x_loc = -1;
+  int enter_loc_fn = -1;
+
+  // irrelevant because they're both defined with [KBD_MATRIX_SZ],
+  // but if they were changed not to:
+  if (sizeof(matrix) != sizeof(matrix_fn)) {
+    fprintf(
+      stderr,
+      "Layer size mismatch! %zu != %zu\n",
+      sizeof(matrix),
+      sizeof(matrix_fn)
+    );
+    return 1;
+  }
+
+  for (i = 0; i < sizeof(matrix); i++) {
+    if (matrix[i] == KEY_COMPOSE) {
+      hyper_loc = i;
+    } else if (matrix[i] == KEY_X) {
+      x_loc = i;
+    }
+  }
+  if (hyper_loc == -1) {
+    fprintf(
+      stderr,
+      "No hyper key (KEY_COMPOSE) found on base layer (matrix)!\n"
+      WHY_HYPER_RETURN
+    );
+    return 1;
+  }
+  if (x_loc == -1) {
+    fprintf(
+      stderr,
+      "No X key (KEY_X) found on base layer (matrix)!\n"
+      WHY_X
+    );
+    return 1;
+  }
+  if (matrix_fn[hyper_loc] != KEY_COMPOSE) {
+    for (i = 0; i < sizeof(matrix_fn); i++) {
+      if (matrix_fn[i] == KEY_COMPOSE) {
+        fprintf(
+          stderr,
+          "Hyper key (KEY_COMPOSE) in different position on layers! %d != %d\n",
+          hyper_loc,
+          i
+        );
+        return 1;
+      }
+    }
+    fprintf(stderr, "No hyper key (KEY_COMPOSE) found on hyper layer (matrix_fn)!\n");
+    return 1;
+  }
+  for (i = 0; i < sizeof(matrix_fn); i++) {
+    if (matrix_fn[i] == KEY_ENTER) {
+      enter_loc_fn = i;
+    }
+  }
+  if (enter_loc_fn == -1) {
+    fprintf(
+      stderr,
+      "No return key (KEY_ENTER) found on hyper layer (matrix_fn)!\n"
+      WHY_HYPER_RETURN
+    );
+    return 1;
+  }
+  return 0;
+}