Index: wp-includes/taxonomy.php
===================================================================
--- wp-includes/taxonomy.php	(revision 18407)
+++ wp-includes/taxonomy.php	(working copy)
@@ -608,6 +608,10 @@
 			'operator' => 'IN',
 		);
 
+		// Single tax queries shouldn't need to pass an array of arrays
+		if( isset( $tax_query['taxonomy'] ) && isset( $tax_query['terms'] ) )
+			$tax_query = array( $tax_query );
+
 		foreach ( $tax_query as $query ) {
 			if ( ! is_array( $query ) )
 				continue;