diff --git a/readthedocs/projects/exceptions.py b/readthedocs/projects/exceptions.py
index eebc24dee38..a37848b3553 100644
--- a/readthedocs/projects/exceptions.py
+++ b/readthedocs/projects/exceptions.py
@@ -38,7 +38,10 @@ class RepositoryError(BuildEnvironmentError):
         'Private repositories are not supported.',
     )
 
-    INVALID_SUBMODULES = _('One or more submodule URLs are not valid: {}.')
+    INVALID_SUBMODULES = _(
+        'One or more submodule URLs are not valid: {}, '
+        'git/ssh URL schemas for submodules are not supported.'
+    )
     INVALID_SUBMODULES_PATH = _(
         'One or more submodule paths are not valid. '
         'Check that all your submodules in .gitmodules are used.'