diff --git a/includes/sanitizers/class-amp-img-sanitizer.php b/includes/sanitizers/class-amp-img-sanitizer.php
index 2253eada51c..4d1511c408c 100644
--- a/includes/sanitizers/class-amp-img-sanitizer.php
+++ b/includes/sanitizers/class-amp-img-sanitizer.php
@@ -84,6 +84,12 @@ private function adjust_and_replace_node( $node ) {
 		$old_attributes = AMP_DOM_Utils::get_node_attributes_as_assoc_array( $node );
 		$new_attributes = $this->filter_attributes( $old_attributes );
 		$new_attributes = $this->enforce_sizes_attribute( $new_attributes );
+		if ( $new_attributes['height'] === '' ) {
+			$new_attributes['height'] = self::FALLBACK_HEIGHT;
+		}
+		if ( $new_attributes['width'] === '' ) {
+			$new_attributes['width'] = self::FALLBACK_WIDTH;
+		}
 		if ( $this->is_gif_url( $new_attributes['src'] ) ) {
 			$this->did_convert_elements = true;
 			$new_tag = 'amp-anim';