diff --git a/src/Document/Base.php b/src/Document/Base.php
index 6589386..bcdb547 100644
--- a/src/Document/Base.php
+++ b/src/Document/Base.php
@@ -103,7 +103,7 @@ protected static function isSupportedUnit($value) {
       'cw',
       'pt',
     );
-    $re = '/^[1-9][0-9]*(' . implode('|', $units) . ')?$/';
+    $re = '/^(0|[1-9][0-9]*)(' . implode('|', $units) . ')?$/';
     return preg_match($re, $value);
   }
 
diff --git a/tests/Document/BaseTest.php b/tests/Document/BaseTest.php
index 131fe11..a887907 100644
--- a/tests/Document/BaseTest.php
+++ b/tests/Document/BaseTest.php
@@ -110,7 +110,7 @@ public function testSetters() {
     }
     foreach (array(
         // Integers.
-        1, 10, 515,
+        0, 1, 10, 515,
         // With unit.
         '1vh', '10vw', '515vmin', '1vmax', '1gut', '1cw', '1pt',
       ) as $value