diff --git a/source/index.ts b/source/index.ts
index afdb3a1..23c1829 100644
--- a/source/index.ts
+++ b/source/index.ts
@@ -136,7 +136,7 @@ namespace is { // tslint:disable-line:no-namespace
 	// tslint:enable:variable-name
 
 	export const nullOrUndefined = (value: any): value is null | undefined => null_(value) || undefined(value);
-	export const numericString = (value: string): bool =>
+	export const numericString = (value: string): boolean =>
 		!Number.isNaN(Number(value));
 
 	export const array = Array.isArray;