diff --git a/internal/config/config.go b/internal/config/config.go
index 269ec8e..95864fa 100644
--- a/internal/config/config.go
+++ b/internal/config/config.go
@@ -113,9 +113,19 @@ func newFromSmartConfig(sc *smartconfig.Config) (*Config, error) {
 			"upstream_connections_per_host", DefaultUpstreamConnectionsPerHost),
 	}
 
-	// Build DBURL from StateDir if not explicitly set
+	// Build DBURL from StateDir if not explicitly set. The derived URL
+	// is a default: it applies only when db_url is omitted, never to an
+	// explicitly empty value.
 	c.DBURL = loader.stringVal("db_url", "")
-	if c.DBURL == "" {
+	if c.DBURL == "" && loader.err == nil {
+		if sc != nil {
+			if _, present := sc.Get("db_url"); present {
+				return nil, fmt.Errorf(
+					"config key %q: value must not be empty; omit the key to derive it from state_dir",
+					"db_url")
+			}
+		}
+
 		c.DBURL = fmt.Sprintf("file:%s/state.sqlite3?_journal_mode=WAL", c.StateDir)
 	}