diff --git a/source/index.ts b/source/index.ts
index 024b4c0..0d86e1f 100644
--- a/source/index.ts
+++ b/source/index.ts
@@ -17,6 +17,11 @@ import {WeakSetPredicate} from './lib/predicates/weak-set';
 
 type TypedArray = Int8Array | Uint8Array | Uint8ClampedArray | Int16Array | Uint16Array | Int32Array | Uint32Array | Float32Array | Float64Array;
 
+/**
+ * @hidden
+ */
+export type Main = <T>(value: T, label: string | Function, predicate: BasePredicate<T>) => void;
+
 export interface Ow {
 	/**
 	 * Test if the value matches the predicate. Throws an `ArgumentError` if the test fails.
@@ -201,23 +206,32 @@ export interface Ow {
 	any(...predicate: BasePredicate[]): AnyPredicate;
 }
 
-const main = <T>(value: T, labelOrPredicate: BasePredicate<T> | string | undefined, predicate?: BasePredicate<T>) => {
-	let label: any = labelOrPredicate;
-	let testPredicate: any = predicate;
+const test = <T>(value: T, label: string | Function, predicate: BasePredicate<T>) => {
+	predicate[testSymbol](value, test, label);
+};
+
+const ow = <T>(value: T, labelOrPredicate: any, predicate?: BasePredicate<T>) => {
+	if (!isPredicate(labelOrPredicate) && typeof labelOrPredicate !== 'string') {
+		throw new TypeError(`Expected second argument to be a predicate or a string, got \`${typeof labelOrPredicate}\``);
+	}
 
 	if (isPredicate(labelOrPredicate)) {
-		label = inferLabel(callsites());
-		testPredicate = labelOrPredicate;
+		// If the second argument is a predicate, infer the label
+		const stackFrames = callsites();
+
+		test(value, () => inferLabel(stackFrames), labelOrPredicate);
+
+		return;
 	}
 
-	return testPredicate[testSymbol](value, main, label);
+	test(value, labelOrPredicate, predicate as BasePredicate<T>);
 };
 
-Object.defineProperties(main, {
+Object.defineProperties(ow, {
 	isValid: {
 		value: <T>(value: T, predicate: BasePredicate<T>) => {
 			try {
-				main(value, predicate);
+				ow(value, predicate);
 				return true;
 			} catch {
 				return false;
@@ -227,10 +241,14 @@ Object.defineProperties(main, {
 	create: {
 		value: <T>(labelOrPredicate: BasePredicate<T> | string | undefined, predicate?: BasePredicate<T>) => (value: T) => {
 			if (isPredicate(labelOrPredicate)) {
-				return main(value, inferLabel(callsites()), labelOrPredicate);
+				const stackFrames = callsites();
+
+				test(value, () => inferLabel(stackFrames), labelOrPredicate);
+
+				return;
 			}
 
-			return main(value, labelOrPredicate, predicate);
+			test(value, labelOrPredicate as string, predicate as BasePredicate<T>);
 		}
 	},
 	any: {
@@ -337,7 +355,7 @@ Object.defineProperties(main, {
 	}
 });
 
-export default main as Ow;
+export default ow as Ow;
 export {
 	BasePredicate,
 	Predicate,
diff --git a/source/lib/predicates/any.ts b/source/lib/predicates/any.ts
index 4d41e66..7aabc1c 100644
--- a/source/lib/predicates/any.ts
+++ b/source/lib/predicates/any.ts
@@ -1,6 +1,6 @@
 import {ArgumentError} from '../argument-error';
 import {BasePredicate, testSymbol} from './base-predicate';
-import {Ow} from '../..';
+import {Main} from '../..';
 
 /**
  * @hidden
@@ -11,14 +11,14 @@ export class AnyPredicate<T = any> implements BasePredicate<T> {
 	) {}
 
 	// tslint:disable completed-docs
-	[testSymbol](value: T, main: Ow, label?: string) {
+	[testSymbol](value: T, main: Main, label: string | Function) {
 		const errors = [
 			'Any predicate failed with the following errors:'
 		];
 
 		for (const predicate of this.predicates) {
 			try {
-				main(value, label as string, predicate);
+				main(value, label, predicate);
 
 				return;
 			} catch (err) {
diff --git a/source/lib/predicates/base-predicate.ts b/source/lib/predicates/base-predicate.ts
index 0f4cede..c9be36e 100644
--- a/source/lib/predicates/base-predicate.ts
+++ b/source/lib/predicates/base-predicate.ts
@@ -1,4 +1,4 @@
-import {Ow} from '../..';
+import {Main} from '../..';
 
 /**
  * @hidden
@@ -15,5 +15,5 @@ export const isPredicate = (value: any): value is BasePredicate => Boolean(value
  */
 export interface BasePredicate<T = any> {
 	// tslint:disable-next-line completed-docs
-	[testSymbol](value: T, main: Ow, label?: string): void;
+	[testSymbol](value: T, main: Main, label: string | Function): void;
 }
diff --git a/source/lib/predicates/predicate.ts b/source/lib/predicates/predicate.ts
index 2d1384b..38adb2e 100644
--- a/source/lib/predicates/predicate.ts
+++ b/source/lib/predicates/predicate.ts
@@ -1,5 +1,5 @@
 import is from '@sindresorhus/is';
-import {Ow} from '../..';
+import {Main} from '../..';
 import {ArgumentError} from '../argument-error';
 import {BasePredicate, testSymbol} from './base-predicate';
 import {not} from '../operators/not';
@@ -55,18 +55,26 @@ export class Predicate<T = any> implements BasePredicate<T> {
 	 * @hidden
 	 */
 	// tslint:disable completed-docs
-	[testSymbol](value: T, main: Ow, label?: string) {
-		const label2 = label
-			? `${this.type} \`${label}\``
-			: this.type;
-
+	[testSymbol](value: T, main: Main, label: string | Function) {
 		for (const {validator, message} of this.context.validators) {
 			const result = validator(value);
 
-			if (typeof result !== 'boolean' || !result) {
-				// TODO: Modify the stack output to show the original `ow()` call instead of this `throw` statement
-				throw new ArgumentError(message(value, label2, result), main);
+			if (result === true) {
+				continue;
+			}
+
+			let label2 = label;
+
+			if (typeof label === 'function') {
+				label2 = label();
 			}
+
+			label2 = label2
+				? `${this.type} \`${label2}\``
+				: this.type;
+
+			// TODO: Modify the stack output to show the original `ow()` call instead of this `throw` statement
+			throw new ArgumentError(message(value, label2, result), main);
 		}
 	}