Index: wp-includes/wp-db.php
===================================================================
--- wp-includes/wp-db.php	(revision 38768)
+++ wp-includes/wp-db.php	(working copy)
@@ -3080,12 +3080,12 @@
 		if ( $this->col_info )
 			return;
 
-		if ( $this->use_mysqli ) {
+		if ( $this->use_mysqli && $this->result instanceof mysqli_result ) {
 			$num_fields = mysqli_num_fields( $this->result );
 			for ( $i = 0; $i < $num_fields; $i++ ) {
 				$this->col_info[ $i ] = mysqli_fetch_field( $this->result );
 			}
-		} else {
+		} elseif ( is_resource( $this->result ) ) {
 			$num_fields = mysql_num_fields( $this->result );
 			for ( $i = 0; $i < $num_fields; $i++ ) {
 				$this->col_info[ $i ] = mysql_fetch_field( $this->result, $i );