diff --git a/src/index.js b/src/index.js
index fd0e1eb..1b1ed59 100644
--- a/src/index.js
+++ b/src/index.js
@@ -110,7 +110,8 @@ function markdown(options = defaultOptions) {
         if (typeof value === 'string') {
           debug.info('Rendering key "%s" of file "%s"', key.join ? key.join('.') : key, file)
           set(data, key, options.render(globalRefsMarkdown + value, options.engineOptions, { path: file, key }))
-        } else {
+          // log a warning if the key is defined and of an unexpected type, but not if the property simply is not defined
+        } else if (typeof value !== 'undefined') {
           debug.warn('Couldn\'t render key %s of file "%s": not a string', key.join ? key.join('.') : key, file)
         }
       })
diff --git a/test/fixtures/default/src/index.md b/test/fixtures/default/src/index.md
index b245991..2594cf5 100644
--- a/test/fixtures/default/src/index.md
+++ b/test/fixtures/default/src/index.md
@@ -1,4 +1,6 @@
-
+---
+not_a_string: false
+---
 # A Markdown Post
 
 With some "amazing", _riveting_, **coooonnnntent**.
\ No newline at end of file