Index: src/wp-includes/customize/class-wp-customize-media-control.php
===================================================================
--- src/wp-includes/customize/class-wp-customize-media-control.php	(revision 37416)
+++ src/wp-includes/customize/class-wp-customize-media-control.php	(working copy)
@@ -214,7 +214,7 @@
 			</div>
 			<div class="actions">
 				<# if ( data.defaultAttachment ) { #>
-					<button type="button" class="button default-button">{{ data.button_labels.default }}</button>
+					<button type="button" class="button default-button">{{ data.button_labels['default'] }}</button>
 				<# } #>
 				<# if ( data.canUpload ) { #>
 				<button type="button" class="button upload-button" id="{{ data.settings['default'] }}-button">{{ data.button_labels.select }}</button>