diff --git a/src/ReferenceListFormatter.php b/src/ReferenceListFormatter.php
index 8f92d3d72..bf93c57aa 100644
--- a/src/ReferenceListFormatter.php
+++ b/src/ReferenceListFormatter.php
@@ -101,7 +101,7 @@ class ReferenceListFormatter {
 			// Make sure the parent is not a subreference.
 			// FIXME: Move to a validation function.
 			$extends =& $ref->extends;
-			if ( $extends !== null && isset( $groupRefs[$extends]->extends ) ) {
+			if ( $extends !== null && isset( $groupRefs[$extends] ) && $groupRefs[$extends]->extends !== null ) {
 				$ref->warnings[] = [ 'cite_error_ref_nested_extends',
 					$extends, $groupRefs[$extends]->extends ];
 			}
diff --git a/src/Validator.php b/src/Validator.php
index 9764b99a0..868568765 100644
--- a/src/Validator.php
+++ b/src/Validator.php
@@ -66,7 +66,7 @@ class Validator {
 			if ( isset( $groupRefs[$name] ) && $groupRefs[$name]->extends === null ) {
 				// T242141: A top-level <ref> can't be changed into a sub-reference
 				return StatusValue::newFatal( 'cite_error_references_duplicate_key', $name );
-			} elseif ( isset( $groupRefs[$extends]->extends ) ) {
+			} elseif ( isset( $groupRefs[$extends] ) && $groupRefs[$extends]->extends !== null ) {
 				// A sub-reference can not be extended a second time (no nesting)
 				return StatusValue::newFatal( 'cite_error_ref_nested_extends', $extends,
 					$groupRefs[$extends]->extends );