diff --git a/doc/external-api/README.md b/doc/external-api/README.md
index 1d7344ddd..8a6f1c085 100644
--- a/doc/external-api/README.md
+++ b/doc/external-api/README.md
@@ -41,6 +41,8 @@ Nix expression `builtins.nixVersion`.
 #include <nix_api_value.h>
 #include <stdio.h>
 
+// NOTE: This example lacks all error handling. Production code must check for
+// errors, as some return values will be undefined.
 int main() {
    nix_libexpr_init(NULL);