diff --git a/source/_docs/configuration/splitting_configuration.markdown b/source/_docs/configuration/splitting_configuration.markdown
index c04bd04460c..2fe7bb652a2 100644
--- a/source/_docs/configuration/splitting_configuration.markdown
+++ b/source/_docs/configuration/splitting_configuration.markdown
@@ -165,7 +165,7 @@ You can get help from the command line using: `hass --script check_config --help
 
 ### {% linkable_title Advanced Usage %}
 
-We offer four advanced options to include whole directories at once.
+We offer four advanced options to include whole directories at once. Please note that your files must have the `.yaml` file extension; `.yml` is not supported.
 - `!include_dir_list` will return the content of a directory as a list with each file content being an entry in the list.
 - `!include_dir_named` will return the content of a directory as a dictionary which maps filename => content of file.
 - `!include_dir_merge_list` will return the content of a directory as a list by merging all files (which should contain a list) into 1 big list.