Index: wp-admin/js/customize-controls.js
===================================================================
--- wp-admin/js/customize-controls.js	(revision 21651)
+++ wp-admin/js/customize-controls.js	(working copy)
@@ -196,8 +196,6 @@
 				}
 			};
 
-			api.UploadControl.prototype.ready.call( this );
-
 			this.thumbnail    = this.container.find('.preview-thumbnail img');
 			this.thumbnailSrc = $.proxy( this.thumbnailSrc, this );
 			this.setting.bind( this.thumbnailSrc );
@@ -245,6 +243,9 @@
 				}
 			});
 
+			if ( this.tabs['upload-new'] )
+				api.UploadControl.prototype.ready.call( this );
+
 			if ( this.tabs.uploaded ) {
 				this.tabs.uploaded.target = this.library.find('.uploaded-target');
 				if ( ! this.tabs.uploaded.panel.find('.thumbnail').length )