make sure that flagVars is not a nil map

command/meta.go

@@ -120,6 +120,9 @@ func (m *Meta) FlagSet(n string, fs FlagSetFlags) *flag.FlagSet {
 		f.Var((*kvflag.Flag)(&m.flagVars), "var", "")
 		f.Var((*kvflag.FlagJSON)(&m.flagVars), "var-file", "")
 	}
+	if len(m.flagVars) == 0 {
+		m.flagVars = make(map[string]string)
+	}
 
 	// Create an io.Writer that writes to our Ui properly for errors.
 	// This is kind of a hack, but it does the job. Basically: create