diff --git a/laser.py b/laser.py
index 446aa45..d971933 100644
--- a/laser.py
+++ b/laser.py
@@ -1357,7 +1357,7 @@ class LaserGcode(inkex.Effect):
             translate = [0, 0]
 
         # doc height in pixels (38 mm == 143.62204724px)
-        doc_height = self.unittouu(self.document.getroot().xpath('@height', namespaces=inkex.NSS)[0])
+        doc_height = self.svg.unittouu(self.document.getroot().xpath('@height', namespaces=inkex.NSS)[0])
 
         if self.document.getroot().get('height') == "100%":
             doc_height = 1052.3622047