diff --git a/src/AmpFormTrait.php b/src/AmpFormTrait.php
index 5503a7b..1c60a0b 100644
--- a/src/AmpFormTrait.php
+++ b/src/AmpFormTrait.php
@@ -159,13 +159,17 @@ trait AmpFormTrait {
    */
   public function validWidth($width, $layout) {
     if (empty($width)) {
-      return 'auto';
+      if ($layout !== 'fill') {
+        return 'auto';
+      }
     }
     switch ($layout) {
       case 'fixed':
       case 'intrinsic':
       case 'responsive':
         return $width;
+      case 'fill':
+        return;
       default:
         return 'auto';
     }
@@ -222,7 +226,9 @@ trait AmpFormTrait {
    */
   public function validHeight($height, $layout) {
     if (empty($height)) {
-      return 'auto';
+      if ($layout !== 'fill') {
+        return 'auto';
+      }
     }
     switch ($layout) {
       case 'fixed':
@@ -230,6 +236,8 @@ trait AmpFormTrait {
       case 'intrinsic':
       case 'responsive':
         return $height;
+      case 'fill':
+        return;
       default:
         return 'auto';
     }