diff --git a/src/Test/Html/Internal/ElmHtml/Query.elm b/src/Test/Html/Internal/ElmHtml/Query.elm
index 64b1c36c..8482e5ec 100644
--- a/src/Test/Html/Internal/ElmHtml/Query.elm
+++ b/src/Test/Html/Internal/ElmHtml/Query.elm
@@ -262,8 +262,25 @@ hasStyle style facts =
 
 classnames : Facts msg -> List String
 classnames facts =
-    Dict.get "className" facts.stringAttributes
-        |> Maybe.withDefault ""
+    (case ( Dict.get "class" facts.stringAttributes, Dict.get "className" facts.stringAttributes ) of
+        ( Just _, Just _ ) ->
+            -- If you use both the `class` attribute and the `className` property at the same time,
+            -- it’s undefined which classes you end up with. It depends on which order they are specified,
+            -- which order elm/virtual-dom happens to apply them, and which of them changed most recently.
+            -- Mixing both is not a good idea. Ideally, we’d show some nice error message explaining this
+            -- here, but since this is very much an edge case it does not feel worth the complexity.
+            -- Instead, silently claim that there are no classes (that no classes match the node).
+            ""
+
+        ( Just class, Nothing ) ->
+            class
+
+        ( Nothing, Just className ) ->
+            className
+
+        ( Nothing, Nothing ) ->
+            ""
+    )
         |> String.split " "
 
 
diff --git a/tests/src/Test/Html/QueryTests.elm b/tests/src/Test/Html/QueryTests.elm
index 134ff5c2..198d7b01 100644
--- a/tests/src/Test/Html/QueryTests.elm
+++ b/tests/src/Test/Html/QueryTests.elm
@@ -113,6 +113,27 @@ all =
                                 [ Query.has [ attribute (Attr.property "className" (Encode.string "hello world")) ]
                                 , Query.has [ attribute (Attr.property "className" (Encode.string "world hello")) ]
                                 ]
+                , test "matches a class added using Attr.attribute" <|
+                    \() ->
+                        divWithAttribute (Attr.attribute "class" "hello")
+                            |> Query.fromHtml
+                            |> Query.has [ class "hello" ]
+                , test "matches a class added using Attr.property" <|
+                    \() ->
+                        divWithAttribute (Attr.property "className" (Encode.string "hello"))
+                            |> Query.fromHtml
+                            |> Query.has [ class "hello" ]
+                , test "matches nothing if classes are added both using Attr.attribute and Attr.property" <|
+                    \() ->
+                        Html.div
+                            [ Attr.attribute "class" "hello"
+                            , Attr.property "className" (Encode.string "world")
+                            ]
+                            []
+                            |> Query.fromHtml
+                            |> Query.has [ class "hello" ]
+                            |> expectationToIsPassing
+                            |> Expect.equal False
                 ]
             ]
         , describe "Query.contains" <|