diff --git a/envconf.go b/envconf.go
index d5ad713..849a968 100644
--- a/envconf.go
+++ b/envconf.go
@@ -259,7 +259,7 @@ func (c *Config) Status() (ok bool) {
 }
 
 func cleanKey(key string) string {
-	expr := regexp.MustCompile("_+")
+	expr := regexp.MustCompile("__+")
 	fn := func(r rune) rune {
 		if (r >= '0' && r <= '9') || (r >= 'A' && r <= 'Z') || r == '_' {
 			return r