diff --git a/src/system/function.ts b/src/system/function.ts
index c5f8166..cb00d1a 100644
--- a/src/system/function.ts
+++ b/src/system/function.ts
@@ -111,16 +111,16 @@ export namespace Functions {
 			: [];
 	}
 
+	export function is<T extends object>(o: T | null | undefined): o is T;
 	export function is<T extends object>(o: object, prop: keyof T, value?: any): o is T;
 	export function is<T extends object>(o: object, matcher: (o: object) => boolean): o is T;
 	export function is<T extends object>(
 		o: object,
-		propOrMatcher: keyof T | ((o: any) => boolean),
+		propOrMatcher?: keyof T | ((o: any) => boolean),
 		value?: any
 	): o is T {
-		if (typeof propOrMatcher === 'function') {
-			return propOrMatcher(o);
-		}
+		if (propOrMatcher == null) return o != null;
+		if (typeof propOrMatcher === 'function') return propOrMatcher(o);
 
 		return value === undefined ? (o as any)[propOrMatcher] !== undefined : (o as any)[propOrMatcher] === value;
 	}