diff --git a/src/FrameDecorator/Page.php b/src/FrameDecorator/Page.php
index 74d4284ad..8f4139075 100644
--- a/src/FrameDecorator/Page.php
+++ b/src/FrameDecorator/Page.php
@@ -515,7 +515,7 @@ function check_page_break(Frame $frame)
         // parents of $frame must fit on the page as well:
         $p = $frame->get_parent();
         while ($p) {
-            $max_y += $p->get_style()->computed_bottom_spacing();
+            $max_y += (float) $p->get_style()->computed_bottom_spacing();
             $p = $p->get_parent();
         }