diff --git a/fs/fs.go b/fs/fs.go
index 02ec37db5..a833a6a07 100644
--- a/fs/fs.go
+++ b/fs/fs.go
@@ -1282,6 +1282,9 @@ type setConfigFile string
 
 // Set a config item into the config file
 func (section setConfigFile) Set(key, value string) {
+	if strings.HasPrefix(string(section), ":") {
+		Errorf(nil, "Can't save config %q = %q for on the fly backend %q", key, value, section)
+	}
 	Debugf(nil, "Saving config %q = %q in section %q of the config file", key, value, section)
 	err := ConfigFileSet(string(section), key, value)
 	if err != nil {