diff --git a/AK/Base64.cpp b/AK/Base64.cpp
index 661d9c53ee..006eeebf1d 100644
--- a/AK/Base64.cpp
+++ b/AK/Base64.cpp
@@ -61,7 +61,7 @@ ByteBuffer decode_base64(const StringView& input)
                 *is_padding = true;
             return 0;
         }
-        return table[input[offset]];
+        return table[static_cast<unsigned char>(input[offset])];
     };
 
     Vector<u8> output;
diff --git a/Tests/AK/TestBase64.cpp b/Tests/AK/TestBase64.cpp
index 3e4baa372b..95a90d06c7 100644
--- a/Tests/AK/TestBase64.cpp
+++ b/Tests/AK/TestBase64.cpp
@@ -27,6 +27,14 @@ TEST_CASE(test_decode)
     decode_equal("Zm9vYmFy", "foobar");
 }
 
+TEST_CASE(test_decode_nocrash)
+{
+    // Any output is fine, we only check that we don't crash here.
+    decode_base64(StringView("asdf\xffqwer"));
+    decode_base64(StringView("asdf\x80qwer"));
+    // TODO: Handle decoding failure.
+}
+
 TEST_CASE(test_encode)
 {
     auto encode_equal = [&](const char* input, const char* expected) {