diff --git a/README.md b/README.md
index f4fcdbc..fd5bcd6 100644
--- a/README.md
+++ b/README.md
@@ -28,7 +28,7 @@ Each of the above sections holds an array of JSON objects with the following dat
 |`isCore`|_Boolean_|Is this considered a direct dependency of `silverstripe/installer`, `silverstripe/recipe-cms` or `silverstripe/recipe-core`? (Only relevant for supported modules)|
 |`lockstepped`|_Boolean_|Whether this is _always_ given a new minor release in "lock step" with Silverstripe CMS as a whole. (Only relevant for supported modules)|
 |`type`|_String_|One of "module", "recipe", "theme", or "other". (Only relevant for supported modules)|
-|`majorVersionMapping`|_Object_|A map of major versions, with the Silverstripe CMS major release lines as object keys and an array of all matching major release lines for the repository as values.<br>• The repository versions are branch names, but in most cases these will map to a major release line (e.g. "5" branch which represents the "5.x" release line)<br>• If a `"*"` key is present, it should be used for any CMS major release lines which do not have their own keys.<br>• If a CMS major release line is missing, and there is no `"*"` key, the repository should be ignored for that CMS major release line.<br>• If the value is an empty array, the default branch should be used.|
+|`majorVersionMapping`|_Object_|A map of major versions, with the Silverstripe CMS major release lines as object keys and an array of all matching major release lines for the repository as values.<br>• The repository versions are branch names, but in most cases these will map to a major release line (e.g. "5" branch which represents the "5.x" release line)<br>• If a `"*"` key is present it will be used for all CMS major release lines, and no other keys can be used<br>• If a CMS major release line is missing, and the `"*"` key is not used, the repository should be ignored for that CMS major release line.<br>• If the value is an empty array, the default branch should be used.|
 
 ## Adding a repo
 
diff --git a/tests/MetaDataTest.php b/tests/MetaDataTest.php
index 1b80e2c..429bc5f 100644
--- a/tests/MetaDataTest.php
+++ b/tests/MetaDataTest.php
@@ -287,5 +287,8 @@ private function validateVersionMap(array $versionMap): void
                 $this->assertNotEmpty($branches);
             }
         }
+        if (array_key_exists('*', $versionMap)) {
+            $this->assertCount(1, array_keys($versionMap), 'If a wildcard is defined, it must be the only key.');
+        }
     }
 }