diff --git a/src/wp-includes/class-wp-query.php b/src/wp-includes/class-wp-query.php
index be646a3268682..1fb08a4520895 100644
--- a/src/wp-includes/class-wp-query.php
+++ b/src/wp-includes/class-wp-query.php
@@ -1212,12 +1212,9 @@ public function parse_tax_query( &$q ) {
 		}
 
 		// If query string 'cat' is an array, implode it.
-		if ( ! is_array( $q['cat'] ) ) {
-			$q['cat'] = explode( ',', $q['cat'] );
-			$q['cat'] = array_map( 'trim', $q['cat'] );
+		if ( is_array( $q['cat'] ) ) {
+			$q['cat'] = implode( ',', $q['cat'] );
 		}
-		sort( $q['cat'] );
-		$q['cat'] = implode( ',', $q['cat'] );
 
 		// Category stuff.
 
@@ -1227,7 +1224,8 @@ public function parse_tax_query( &$q ) {
 
 			$cat_array = preg_split( '/[,\s]+/', urldecode( $q['cat'] ) );
 			$cat_array = array_map( 'intval', $cat_array );
-			$q['cat']  = implode( ',', $cat_array );
+			sort( $cat_array );
+			$q['cat'] = implode( ',', $cat_array );
 
 			foreach ( $cat_array as $cat ) {
 				if ( $cat > 0 ) {
diff --git a/tests/phpunit/tests/query/parseQuery.php b/tests/phpunit/tests/query/parseQuery.php
index bbf3f1217fb2e..94ced1ecd6e75 100644
--- a/tests/phpunit/tests/query/parseQuery.php
+++ b/tests/phpunit/tests/query/parseQuery.php
@@ -151,7 +151,7 @@ public function test_parse_query_cat_array_mixed() {
 			)
 		);
 
-		$this->assertSame( '1,-1', $q->query_vars['cat'] );
+		$this->assertSame( '-1,1', $q->query_vars['cat'] );
 	}
 
 	/**