diff --git a/includes/libs/http/HttpAcceptParser.php b/includes/libs/http/HttpAcceptParser.php
index 625237c6d39..625ad6ef9d5 100644
--- a/includes/libs/http/HttpAcceptParser.php
+++ b/includes/libs/http/HttpAcceptParser.php
@@ -40,7 +40,11 @@ class HttpAcceptParser {
 			if ( isset( $matches[3] ) ) {
 				$kvps = explode( ';', $matches[3] );  // FIXME: Allow semi-colon in quotes
 				foreach ( $kvps as $kv ) {
-					[ $key, $val ] = explode( '=', trim( $kv ), 2 );
+					$kvArray = explode( '=', trim( $kv ), 2 );
+					if ( count( $kvArray ) != 2 ) {
+						continue;
+					}
+					[ $key, $val ] = $kvArray;
 					$key = strtolower( trim( $key ) );
 					$val = trim( $val );
 					if ( $key === 'q' ) {
diff --git a/tests/phpunit/unit/includes/libs/http/HttpAcceptParserTest.php b/tests/phpunit/unit/includes/libs/http/HttpAcceptParserTest.php
index d930a8855e1..31089774340 100644
--- a/tests/phpunit/unit/includes/libs/http/HttpAcceptParserTest.php
+++ b/tests/phpunit/unit/includes/libs/http/HttpAcceptParserTest.php
@@ -139,6 +139,26 @@ class HttpAcceptParserTest extends TestCase {
 					]
 				]
 			],
+			[
+				// Incomplete q - T391867
+				'test/123; q=0.5, test/789; q',
+				[
+					[
+						'type' => 'test',
+						'subtype' => '789',
+						'q' => 1,
+						'i' => 1,
+						'params' => []
+					],
+					[
+						'type' => 'test',
+						'subtype' => '123',
+						'q' => 0.5,
+						'i' => 0,
+						'params' => []
+					],
+				]
+			],
 		];
 	}