diff --git a/config.md b/config.md
index efaf0a0e5..8ed840406 100644
--- a/config.md
+++ b/config.md
@@ -260,6 +260,8 @@ _Note: symbolic name for uid and gid, such as uname and gname respectively, are
 For Windows based systems the user structure has the following fields:
 
 * **`username`** (string, OPTIONAL) specifies the user name for the process.
+    The value MUST NOT be an empty string.
+    The default `username` is implementation-defined.
 
 ### Example (Windows)
 
diff --git a/schema/config-schema.json b/schema/config-schema.json
index 87e46dfef..ebba3fcb6 100644
--- a/schema/config-schema.json
+++ b/schema/config-schema.json
@@ -130,6 +130,11 @@
                         "additionalGids": {
                             "id": "https://opencontainers.org/schema/bundle/process/user/additionalGids",
                             "$ref": "defs.json#/definitions/ArrayOfGIDs"
+                        },
+                        "username": {
+                            "id": "https://opencontainers.org/schema/bundle/process/user/username",
+                            "type": "string",
+                            "minLength": 1
                         }
                     }
                 },