diff --git a/i18n/en.json b/i18n/en.json
index e82564074..c4f70ad34 100644
--- a/i18n/en.json
+++ b/i18n/en.json
@@ -21,6 +21,7 @@
 	"cite_error_ref_numeric_key": "Invalid <code>&lt;ref&gt;</code> tag;\nname cannot be a simple integer. Use a descriptive title",
 	"cite_error_ref_no_key": "The opening <code>&lt;ref&gt;</code> tag is malformed or has a bad name",
 	"cite_error_ref_too_many_keys": "Invalid <code>&lt;ref&gt;</code> tag;\ninvalid names, e.g. too many",
+	"cite_error_ref_nested_extends": "Extending <code>&lt;ref&gt;</code> tags more than one level deep is not allowed",
 	"cite_error_ref_no_input": "Invalid <code>&lt;ref&gt;</code> tag;\nrefs with no name must have content",
 	"cite_error_references_duplicate_key": "Invalid <code>&lt;ref&gt;</code> tag; name \"$1\" defined multiple times with different content",
 	"cite_error_references_invalid_parameters": "Invalid parameter in <code>&lt;references&gt;</code> tag",
diff --git a/i18n/qqq.json b/i18n/qqq.json
index d9f1d3725..ea728ebdb 100644
--- a/i18n/qqq.json
+++ b/i18n/qqq.json
@@ -22,6 +22,7 @@
 	"cite_error_ref_numeric_key": "Error message shown if the name of a ref tag only contains digits. Examples that cause this error are <code>&lt;ref name=\"123\"/&gt;</code> or <code>&lt;ref name=\"456\">input</ref&gt;</code>",
 	"cite_error_ref_no_key": "Error message shown when ref tags without any content (that is <code>&lt;ref/&gt;</code>) are used without a name.",
 	"cite_error_ref_too_many_keys": "Error message shown when ref tags has parameters other than name and group. Examples that cause this error are <code>&lt;ref name=\"name\" notname=\"value\"/&gt;</code> or <code>&lt;ref notname=\"value\" >input<ref&gt;</code>",
+	"cite_error_ref_nested_extends": "Error message shown when a ref tag tries to extend another ref tag that's already extended, i.e. nesting sub-refs is not allowed.\n\nParameters:\n* $1 - Name of the invalid ref\n* $2 - Name of the previous, conflicting ref",
 	"cite_error_ref_no_input": "Error message shown when ref tags without names have no content. An example that cause this error is <code>&lt;ref></ref&gt;</code>",
 	"cite_error_references_duplicate_key": "Error message shown when multiple refs with same name exist but with different content",
 	"cite_error_references_invalid_parameters": "Error message shown when parameters are used in the references tag. An example that causes this error is <code>&lt;references someparameter=\"value\"/&gt;</code>.",
diff --git a/src/Cite.php b/src/Cite.php
index 1c2f5d3aa..e0097eb1f 100644
--- a/src/Cite.php
+++ b/src/Cite.php
@@ -180,8 +180,8 @@ class Cite {
 				return StatusValue::newFatal( 'cite_error_references_duplicate_key', $name );
 			} elseif ( isset( $groupRefs[$extends]['extends'] ) ) {
 				// A sub-reference can not be extended a second time (no nesting)
-				// TODO: Introduce a specific error for this case, reuse in formatReferences()!
-				return StatusValue::newFatal( 'cite_error_ref_too_many_keys' );
+				return StatusValue::newFatal( 'cite_error_ref_nested_extends', $extends,
+					$groupRefs[$extends]['extends'] );
 			}
 		}
 
diff --git a/src/ReferencesFormatter.php b/src/ReferencesFormatter.php
index 354ca6e97..0e18334fc 100644
--- a/src/ReferencesFormatter.php
+++ b/src/ReferencesFormatter.php
@@ -119,8 +119,8 @@ class ReferencesFormatter {
 				isset( $groupRefs[$value['extends']]['extends'] )
 			) {
 				$value['text'] = ( $value['text'] ?? '' ) . ' ' .
-					// TODO: Introduce a specific error for this case; reuse in validateRef()!
-					$this->errorReporter->plain( $parser, 'cite_error_ref_too_many_keys' );
+					$this->errorReporter->plain( $parser, 'cite_error_ref_nested_extends',
+						$value['extends'], $groupRefs[$value['extends']]['extends'] );
 			}
 
 			if ( !$indented && isset( $value['extends'] ) ) {
diff --git a/tests/parser/bookReferencing.txt b/tests/parser/bookReferencing.txt
index daa626698..4ffaa0708 100644
--- a/tests/parser/bookReferencing.txt
+++ b/tests/parser/bookReferencing.txt
@@ -243,8 +243,7 @@ wgCiteBookReferencing=true
 !! html/php
 <p><sup id="cite_ref-a_1-0" class="reference"><a href="#cite_note-a-1">&#91;1&#93;</a></sup>
 <sup id="cite_ref-b_2-0" class="reference"><a href="#cite_note-b-2">&#91;1.1&#93;</a></sup>
-<span class="error mw-ext-cite-error" lang="en" dir="ltr">Cite error: Invalid <code>&lt;ref&gt;</code> tag;
-invalid names, e.g. too many</span>
+<span class="error mw-ext-cite-error" lang="en" dir="ltr">Cite error: Extending <code>&lt;ref&gt;</code> tags more than one level deep is not allowed</span>
 </p>
 <div class="mw-references-wrap"><ol class="references">
 <li id="cite_note-a-1"><span class="mw-cite-backlink"><a href="#cite_ref-a_1-0">↑</a></span> <span class="reference-text">book</span>
diff --git a/tests/phpunit/unit/ReferencesFormatterTest.php b/tests/phpunit/unit/ReferencesFormatterTest.php
index 3c34285ca..0efb297b2 100644
--- a/tests/phpunit/unit/ReferencesFormatterTest.php
+++ b/tests/phpunit/unit/ReferencesFormatterTest.php
@@ -126,7 +126,7 @@ class ReferencesFormatterTest extends \MediaWikiUnitTestCase {
 				'<div class="mw-references-wrap"><ol class="references">' . "\n" .
 					'<li>(cite_references_link_many|||<span class="reference-text">t3</span>' . "\n" .
 					'|)<ol class="mw-extended-references"><li>(cite_references_link_many|||' .
-					'<span class="reference-text">t1 (cite_error_ref_too_many_keys)</span>' .
+					'<span class="reference-text">t1 (cite_error_ref_nested_extends|a|b)</span>' .
 					"\n|)</li>\n" .
 					'<li>(cite_references_link_many|||<span class="reference-text">t2</span>' .
 					"\n|)</li>\n</ol></li>\n" .