diff --git a/tests/src/isCreditCardValid.test.ts b/tests/src/isCreditCardValid.test.ts
index 1b289c9..5850513 100644
--- a/tests/src/isCreditCardValid.test.ts
+++ b/tests/src/isCreditCardValid.test.ts
@@ -12,9 +12,88 @@ describe("isCreditCardValid", () => {
 		expect(result).toBe(false);
 	});
 
+	it("should return false when the length of the input is less than 13", () => {
+		const result = isCreditCardValid("123456781234");
+		expect(result).toBe(false);
+	});
+
+	it("should return false when the length of the input is greater than 19", () => {
+		const result = isCreditCardValid("12345678123456781234");
+		expect(result).toBe(false);
+	});
+
 	it("should throw an error when the input is not a string", () => {
 		expect(() => isCreditCardValid(1234567812345678 as any)).toThrow(
 			"The input should be a string.",
 		);
 	});
+
+	it("should return false when the input is an empty string", () => {
+		const result = isCreditCardValid("");
+		expect(result).toBe(false);
+	});
+
+	it("should return false when the input is a string with only spaces", () => {
+		const result = isCreditCardValid("    ");
+		expect(result).toBe(false);
+	});
+
+	it("should return false when the input is a string with only letters", () => {
+		const result = isCreditCardValid("abcdefgh");
+		const result2 = isCreditCardValid("asdfghjklcmvnshdg==");
+		const result3 = isCreditCardValid("asdfghjklcmvnshdg");
+		expect(result).toBe(false);
+		expect(result2).toBe(false);
+		expect(result3).toBe(false);
+	});
+
+	it("should return false when having a letter in the middle of the string", () => {
+		const result = isCreditCardValid("1234a5678");
+		expect(result).toBe(false);
+	});
+
+	it("should return false when the input is a string with only special characters", () => {
+		const result = isCreditCardValid("++++");
+		expect(result).toBe(false);
+	});
+
+	it("should return false when the input is a string with only special characters and numbers", () => {
+		const result = isCreditCardValid("1234++++");
+		expect(result).toBe(false);
+	});
+
+	it("should return false when the input is a string with only special characters and letters", () => {
+		const result = isCreditCardValid("abcd++++");
+		expect(result).toBe(false);
+	});
+
+	it("should return false when the input is a string with only special characters, letters and numbers", () => {
+		const result = isCreditCardValid("abcd1234++++");
+		expect(result).toBe(false);
+	});
+
+	it("should return false when the input is a string with only special characters and spaces", () => {
+		const result = isCreditCardValid("    ++++");
+		expect(result).toBe(false);
+	});
+
+	it("should return false when the input is a string with only special characters, spaces and numbers", () => {
+		const result = isCreditCardValid("1234    ++++");
+		expect(result).toBe(false);
+	});
+
+	it("should return false when the input is a string with only special characters, spaces and letters", () => {
+		const result = isCreditCardValid("abcd    ++++");
+		expect(result).toBe(false);
+	});
+
+	it("should return false when the input is a string with only special characters, spaces, letters and numbers", () => {
+		const result = isCreditCardValid("abcd1234    ++++");
+		expect(result).toBe(false);
+	});
+
+	it("should return false when the input is a string with only spaces and letters", () => {
+		const result = isCreditCardValid("    abcd");
+		expect(result).toBe(false);
+	});
 });