diff --git a/docs/content/2.guide/2.features/10.runtime-config.md b/docs/content/2.guide/2.features/10.runtime-config.md
index 701f3cfd28..634e59612e 100644
--- a/docs/content/2.guide/2.features/10.runtime-config.md
+++ b/docs/content/2.guide/2.features/10.runtime-config.md
@@ -37,7 +37,7 @@ Nuxt CLI has built-in [dotenv](https://github.com/motdotla/dotenv) support.
 
 In addition to any process environment variables, if you have a `.env` file in your project root directory, it will be automatically loaded into `process.env` and accessible within your `nuxt.config` file and modules.
 
-Runtime config values are automatically replaced by matching environment variables at runtime.
+Runtime config values are automatically replaced by matching environment variables at runtime. For this to work, you _must_ have a fallback value (which can just be an empty string) defined in your `nuxt.config`.
 
 **Example:**