diff --git a/CHANGES.md b/CHANGES.md
index 8e061c7f..0164a489 100644
--- a/CHANGES.md
+++ b/CHANGES.md
@@ -2,6 +2,10 @@
 
 ## 3.5.1
 
+Fixes:
+
+- Correctly sort YAML map keys with mixed types. (#433, #441)
+
 ## 3.5.0 (2024-04-29)
 
 Development:
diff --git a/markdown.dtx b/markdown.dtx
index bbbd06e8..dcc8e6af 100644
--- a/markdown.dtx
+++ b/markdown.dtx
@@ -32359,13 +32359,38 @@ M.extensions.jekyll_data = function(expect_jekyll_data)
         for k, _ in pairs(d) do
           table.insert(keys, k)
         end
-        table.sort(keys)
+%    \end{macrocode}
+% \begin{markdown}
+%
+% For reproducibility, sort the keys. For mixed string-and-numeric keys, sort
+% numeric keys before string keys.
+%
+% \end{markdown}
+%  \begin{macrocode}
+        table.sort(keys, function(first, second)
+          if type(first) ~= type(second) then
+            return type(first) < type(second)
+          else
+            return first < second
+          end
+        end)
 
         if not p then
           table.insert(buf, "\\markdownRendererJekyllDataBegin")
         end
 
-        if #d > 0 then
+        local is_sequence = false
+        if #d > 0 and #d == #keys then
+          for i=1, #d do
+            if d[i] == nil then
+              goto not_a_sequence
+            end
+          end
+          is_sequence = true
+        end
+        ::not_a_sequence::
+
+        if is_sequence then
             table.insert(buf, "\\markdownRendererJekyllDataSequenceBegin{")
             table.insert(buf, self.identifier(p or "null"))
             table.insert(buf, "}{")
@@ -32420,7 +32445,7 @@ M.extensions.jekyll_data = function(expect_jekyll_data)
           end
         end
 
-        if #d > 0 then
+        if is_sequence then
           table.insert(buf, "\\markdownRendererJekyllDataSequenceEnd")
         else
           table.insert(buf, "\\markdownRendererJekyllDataMappingEnd")
diff --git a/tests/testfiles/lunamark-markdown/jekyll-data-mixed-keys.test b/tests/testfiles/lunamark-markdown/jekyll-data-mixed-keys.test
new file mode 100644
index 00000000..e782a6b3
--- /dev/null
+++ b/tests/testfiles/lunamark-markdown/jekyll-data-mixed-keys.test
@@ -0,0 +1,32 @@
+\markdownSetup{jekyllData=true}
+<<<
+This test ensures that the Lua `jekyllData` option correctly propagates
+through the plain TeX interface and that mixed numeric and string keys
+are correctly sorted.
+
+---
+1: foo
+a: bar
+---
+>>>
+BEGIN document
+codeSpan: jekyllData
+softLineBreak
+softLineBreak
+interblockSeparator
+jekyllDataBegin
+BEGIN jekyllDataMappingBegin
+- key: null
+- length: 2
+END jekyllDataMappingBegin
+BEGIN jekyllDataString
+- key: 1
+- value: foo
+END jekyllDataString
+BEGIN jekyllDataString
+- key: a
+- value: bar
+END jekyllDataString
+jekyllDataMappingEnd
+jekyllDataEnd
+END document