diff --git a/.travis.yml b/.travis.yml
index 6d64d6a..2e478e7 100644
--- a/.travis.yml
+++ b/.travis.yml
@@ -5,6 +5,7 @@
 
 dart_task:
   - test: --platform vm,chrome
+  - script: pub run --enable-experiment=non-nullable test --platform vm,chrome
 
 matrix:
   include:
diff --git a/lib/src/tree.dart b/lib/src/tree.dart
index 2e987ce..ab4092c 100644
--- a/lib/src/tree.dart
+++ b/lib/src/tree.dart
@@ -1583,7 +1583,7 @@
       String? variant,
       LineHeight? lineHeight})
       : font = Font(
-            size: size is LengthTerm ? size.value : size as num,
+            size: (size is LengthTerm ? size.value : size) as num?,
             family: family,
             weight: weight,
             style: style,