diff --git a/source/index.ts b/source/index.ts
index 3f3f1e5..36004d2 100644
--- a/source/index.ts
+++ b/source/index.ts
@@ -487,7 +487,7 @@ interface Assert {
 	truthy: (value: unknown) => asserts value is unknown;
 	falsy: (value: unknown) => asserts value is unknown;
 	nan: (value: unknown) => asserts value is unknown;
-	primitive: (value: unknown) => asserts value is unknown;
+	primitive: (value: unknown) => asserts value is Primitive;
 	integer: (value: unknown) => asserts value is number;
 	safeInteger: (value: unknown) => asserts value is number;
 	plainObject: (value: unknown) => asserts value is {[key: string]: unknown};
@@ -575,7 +575,7 @@ export const assert: Assert = {
 	truthy: (value: unknown): asserts value is unknown => assertType(is.truthy(value), AssertionTypeDescription.truthy, value),
 	falsy: (value: unknown): asserts value is unknown => assertType(is.falsy(value), AssertionTypeDescription.falsy, value),
 	nan: (value: unknown): asserts value is unknown => assertType(is.nan(value), AssertionTypeDescription.nan, value),
-	primitive: (value: unknown): asserts value is unknown => assertType(is.primitive(value), AssertionTypeDescription.primitive, value),
+	primitive: (value: unknown): asserts value is Primitive => assertType(is.primitive(value), AssertionTypeDescription.primitive, value),
 	integer: (value: unknown): asserts value is number => assertType(is.integer(value), AssertionTypeDescription.integer, value),
 	safeInteger: (value: unknown): asserts value is number => assertType(is.safeInteger(value), AssertionTypeDescription.safeInteger, value),
 	plainObject: (value: unknown): asserts value is {[key: string]: unknown} => assertType(is.plainObject(value), AssertionTypeDescription.plainObject, value),