diff --git a/internal/config/config.go b/internal/config/config.go
index 850e17d..5452e33 100644
--- a/internal/config/config.go
+++ b/internal/config/config.go
@@ -108,7 +108,7 @@ func (c *Config) Validate() error {
 			if a == "" {
 				return fmt.Errorf("aggregate.include_aliases[%d]: empty entry", i)
 			}
-			if !seen[a] {
+			if _, ok := seen[a]; !ok {
 				return fmt.Errorf("aggregate.include_aliases[%d]: unknown server alias %q", i, a)
 			}
 		}