diff --git a/src/hocon_pp.erl b/src/hocon_pp.erl
index 3f0cd5b..858182e 100644
--- a/src/hocon_pp.erl
+++ b/src/hocon_pp.erl
@@ -332,6 +332,10 @@ maybe_quote_key(K0) ->
             K0
     end.
 
+is_quote_key("null") ->
+    %% `null' is a primitive value in hocon, so if a `"null"' string is used as a map key,
+    %% it must be quoted to avoid confusion with the `null' primitive.
+    true;
 is_quote_key(K) ->
     case io_lib:printable_latin1_list(K) of
         true ->
diff --git a/test/hocon_pp_tests.erl b/test/hocon_pp_tests.erl
index 58f643e..8e66324 100644
--- a/test/hocon_pp_tests.erl
+++ b/test/hocon_pp_tests.erl
@@ -358,3 +358,21 @@ crlf_multiline_test_() ->
         ?_assertEqual({ok, Value}, hocon:binary(Expected)),
         ?_assertEqual({ok, Value}, hocon:binary(Variant))
     ].
+
+%% Checks that if the string `"null"' is used as a map key, it's correctly serialized and
+%% then deserialized.
+null_map_key_test_() ->
+    Roundtrip = fun(NVal) ->
+        Value = #{<<"root">> => #{NVal => 1}},
+        IOList = hocon_pp:do(Value, #{}),
+        ?assertEqual({ok, Value}, hocon:binary(IOList))
+    end,
+    [
+        ?_test(Roundtrip(Value))
+     || Value <- [
+            <<"null">>,
+            <<"Null">>,
+            <<"NULL">>,
+            <<"nUlL">>
+        ]
+    ].