diff --git a/axe.d.ts b/axe.d.ts
index 88bc95a725..d6f55278a0 100644
--- a/axe.d.ts
+++ b/axe.d.ts
@@ -3,7 +3,7 @@
 // Definitions by: Marcy Sutton <https://github.com/marcysutton>
 
 declare namespace axe {
-	type ImpactValue = 'minor' | 'moderate' | 'serious' | 'critical';
+	type ImpactValue = 'minor' | 'moderate' | 'serious' | 'critical' | null;
 
 	type TagValue = 'wcag2a' | 'wcag2aa' | 'section508' | 'best-practice' | 'wcag21a' | 'wcag21aa';