diff --git a/hmac.cpp b/hmac.cpp
index 319ff8d..eec053e 100644
--- a/hmac.cpp
+++ b/hmac.cpp
@@ -102,7 +102,7 @@ namespace hmac {
                 digest_size = hmac_hash::SHA512::DIGEST_SIZE;
                 break;
             default:
-                return {};
+                throw std::invalid_argument("Unsupported hash type");
         }
 
         // Step 1: Normalize key
diff --git a/test_all.cpp b/test_all.cpp
index 91e8777..7e2254a 100644
--- a/test_all.cpp
+++ b/test_all.cpp
@@ -51,6 +51,13 @@ TEST(HMACTest, NullPointersThrow) {
     EXPECT_THROW(hmac::get_hmac(key, 3, nullptr, 1, hmac::TypeHash::SHA256), std::invalid_argument);
 }
 
+TEST(HMACTest, InvalidTypeThrows) {
+    const char* key = "key";
+    const char* msg = "abc";
+    auto invalid = static_cast<hmac::TypeHash>(999);
+    EXPECT_THROW(hmac::get_hmac(key, 3, msg, 3, invalid), std::invalid_argument);
+}
+
 TEST(TOTPTest, AtTime) {
     const std::string totp_key = "12345678901234567890";
     uint64_t test_time = 1234567890;