diff --git a/radicale/sharing/__init__.py b/radicale/sharing/__init__.py
index 551450cd..0c010330 100644
--- a/radicale/sharing/__init__.py
+++ b/radicale/sharing/__init__.py
@@ -947,7 +947,7 @@ class BaseSharing:
             Conversion = request_data['Conversion']
             # verify against whitelist
             if Conversion not in CONVERSIONS_WHITELIST:
-                return httputils.bad_request("Conversion not supported: %r" % Conversion)
+                return httputils.bad_request("Conversion is not supported: %r" % Conversion)
 
         if 'Actions' in request_data:
             valid = True  # default
@@ -1060,11 +1060,11 @@ class BaseSharing:
                     return httputils.NOT_FOUND
                 if not isinstance(item, storage.BaseCollection):
                     logger.warning(api_info + ": PathMapped=%r is not a collection", PathMapped)
-                    return httputils.METHOD_NOT_ALLOWED
+                    return httputils.bad_request("PathMapped is not a collection")
                 if Conversion == "bday":
                     if item.tag != "VADDRESSBOOK":
                         logger.warning(api_info + ": PathMapped=%r is not a VADDRESSBOOK collection (mandatory for Conversion=%r)", PathMapped, Conversion)
-                        return httputils.METHOD_NOT_ALLOWED
+                        return httputils.bad_request("Conversion is not supported for collection type")
 
             if Permissions is None:
                 if ShareType == "token":
@@ -1081,7 +1081,7 @@ class BaseSharing:
                     for permission in Permissions:
                         if permission not in "rPp":
                             logger.warning(api_info + ": PathMapped=%r Permissions=%r not supported for Conversion=%r", PathMapped, Permissions, Conversion)
-                            return httputils.METHOD_NOT_ALLOWED
+                            return httputils.bad_request("Permissions are not supported for conversion")
 
             if Enabled is None:
                 Enabled = False # security by default
@@ -1299,7 +1299,7 @@ class BaseSharing:
                     for permission in Permissions:
                         if permission not in "rPp":
                             logger.warning(api_info + ": PathMapped=%r Permissions=%r not supported for Conversion=%r", PathMapped, Permissions, Conversion)
-                            return httputils.METHOD_NOT_ALLOWED
+                            return httputils.bad_request("Permissions are not supported for conversion")
 
             if user == share['Owner']:
                 if PathMapped is not None: