diff --git a/blake2/src/macros.rs b/blake2/src/macros.rs
index ea0f26532..e0277c4d8 100644
--- a/blake2/src/macros.rs
+++ b/blake2/src/macros.rs
@@ -353,7 +353,7 @@ macro_rules! blake2_mac_impl {
             #[inline]
             fn new_from_slice(key: &[u8]) -> Result<Self, InvalidLength> {
                 let kl = key.len();
-                if kl > <Self as KeySizeUser>::KeySize::USIZE {
+                if kl == 0 || kl > <Self as KeySizeUser>::KeySize::USIZE {
                     return Err(InvalidLength);
                 }
                 let mut padded_key = Block::<$hash>::default();
diff --git a/blake2/tests/mac.rs b/blake2/tests/mac.rs
index 2deca402f..69e47f0ec 100644
--- a/blake2/tests/mac.rs
+++ b/blake2/tests/mac.rs
@@ -32,6 +32,10 @@ fn blake2b_new_test() {
 
 #[test]
 fn mac_refuses_empty_keys() {
+    use blake2::digest::KeyInit;
+
+    assert!(blake2::Blake2bMac512::new_from_slice(&[]).is_err());
+    assert!(blake2::Blake2sMac256::new_from_slice(&[]).is_err());
     assert!(
         blake2::Blake2bMac512::new_with_salt_and_personal(Some(&[]), b"salt", b"persona").is_err()
     );