diff --git a/includes/GlobalFunctions.php b/includes/GlobalFunctions.php
index 61bf30318f8..d96b17f3ffe 100644
--- a/includes/GlobalFunctions.php
+++ b/includes/GlobalFunctions.php
@@ -399,7 +399,7 @@ function wfCgiToArray( $query ) {
 				$k = substr( $k, 0, -1 );
 				$temp = [ $k => $temp ];
 			}
-			if ( isset( $ret[$key] ) ) {
+			if ( isset( $ret[$key] ) && is_array( $ret[$key] ) ) {
 				$ret[$key] = array_merge( $ret[$key], $temp );
 			} else {
 				$ret[$key] = $temp;
diff --git a/tests/phpunit/includes/GlobalFunctions/GlobalTest.php b/tests/phpunit/includes/GlobalFunctions/GlobalTest.php
index 5f2cc5804d5..4a08613a128 100644
--- a/tests/phpunit/includes/GlobalFunctions/GlobalTest.php
+++ b/tests/phpunit/includes/GlobalFunctions/GlobalTest.php
@@ -192,7 +192,8 @@ class GlobalTest extends MediaWikiIntegrationTestCase {
 			[ 'foo', [ 'foo' => '' ] ], // missing =
 			[ 'foo=bar&qwerty=asdf', [ 'foo' => 'bar', 'qwerty' => 'asdf' ] ], // multiple value
 			[ 'foo=A%26B%3D5%2B6%40%21%22%27', [ 'foo' => 'A&B=5+6@!"\'' ] ], // urldecoding test
-			[ 'foo%5Bbar%5D=baz', [ 'foo' => [ 'bar' => 'baz' ] ] ],
+			[ 'foo[bar]=baz', [ 'foo' => [ 'bar' => 'baz' ] ] ],
+			[ 'foo%5Bbar%5D=baz', [ 'foo' => [ 'bar' => 'baz' ] ] ],  // urldecoding test 2
 			[
 				'foo%5Bbar%5D=baz&foo%5Bqwerty%5D=asdf',
 				[ 'foo' => [ 'bar' => 'baz', 'qwerty' => 'asdf' ] ]
@@ -202,6 +203,8 @@ class GlobalTest extends MediaWikiIntegrationTestCase {
 				'foo%5Bbar%5D%5Bbar%5D=baz',
 				[ 'foo' => [ 'bar' => [ 'bar' => 'baz' ] ] ]
 			],
+			[ 'foo[]=x&foo[]=y', [ 'foo' => [ '' => 'y' ] ] ], // implicit keys are NOT handled like in PHP (bug?)
+			[ 'foo=x&foo[]=y', [ 'foo' => [ '' => 'y' ] ] ], // mixed value/array doesn't cause errors
 		];
 	}