diff --git a/Userland/Libraries/LibCrypto/Authentication/HMAC.h b/Userland/Libraries/LibCrypto/Authentication/HMAC.h
index 29e353c6e8b..292c8a30084 100644
--- a/Userland/Libraries/LibCrypto/Authentication/HMAC.h
+++ b/Userland/Libraries/LibCrypto/Authentication/HMAC.h
@@ -98,7 +98,7 @@ private:
             auto digest = m_inner_hasher.digest();
             // FIXME: should we check if the hash function creates more data than its block size?
             key_buffer.overwrite(0, digest.immutable_data(), m_inner_hasher.digest_size());
-        } else {
+        } else if (length > 0) {
             key_buffer.overwrite(0, key, length);
         }