Index: src/wp-includes/js/quicktags.js
===================================================================
--- src/wp-includes/js/quicktags.js	(revision 31068)
+++ src/wp-includes/js/quicktags.js	(working copy)
@@ -260,8 +260,10 @@
 			use = '';
 
 			// set buttons
-			if ( settings.buttons ) {
+			if ( typeof settings.buttons === 'string' ) {
 				use = ','+settings.buttons+',';
+			} else if ( settings.buttons === false ) {
+				use = ' '; // Any old truthy string.
 			}
 
 			for ( i in edButtons ) {