Index: src/wp-includes/wp-db.php
===================================================================
--- src/wp-includes/wp-db.php	(revision 33703)
+++ src/wp-includes/wp-db.php	(working copy)
@@ -2401,7 +2401,7 @@
 		}
 
 		// Skip this entirely if this isn't a MySQL database.
-		if ( false === $this->is_mysql ) {
+		if ( false === $this->is_mysql || empty( $this->is_mysql ) ) {
 			return false;
 		}
 
@@ -2450,7 +2450,7 @@
 		$columnkey = strtolower( $column );
 
 		// Skip this entirely if this isn't a MySQL database.
-		if ( false === $this->is_mysql ) {
+		if ( false === $this->is_mysql || empty( $this->is_mysql ) ) {
 			return false;
 		}