diff --git a/envconf.go b/envconf.go
index 3bb6abc..4697c83 100644
--- a/envconf.go
+++ b/envconf.go
@@ -84,8 +84,8 @@ func (c *Config) Define(key string, dtype DataType) {
 // Define the type of an environment variable.
 // Variables without a defined type will be ignored by Parse.
 func (c *Config) DefineMap(key string, dtype DataType) {
-	upper := strings.ToUpper(key)
-	entries, ok := c.mapEnv[upper]
+	key = cleanKey(key)
+	entries, ok := c.mapEnv[key]
 	if ok {
 		for mapKey, entry := range entries {
 			entry.dtype = dtype