config: only walk if non-nil

config/config.go

@@ -127,6 +127,7 @@ func (c *Config) Validate() error {
 		}
 
 		w := &interpolationWalker{F: fn}
+		if v.Default != nil {
 			if err := reflectwalk.Walk(v.Default, w); err == nil {
 				if interp {
 					errs = append(errs, fmt.Errorf(
@@ -135,6 +136,7 @@ func (c *Config) Validate() error {
 				}
 			}
 		}
+	}
 
 	// Check for references to user variables that do not actually
 	// exist and record those errors.