Why didn’t Romania join the invasion of Czechoslovakia of 1968?

Why didn’t Romania join the invasion of Czechoslovakia of 1968? As the Soviet Union worked tirelessly to keep the Iron Curtain up and strong after the second world war, Czechoslovakia unintentionally became a threat to this objective. As a satellite state of the Union, Czechoslovakia was meant to remain a firm communist-led country, just like the rest of the Eastern Bloc. So, when a new Czech government began a process of reform known as the “Prague Spring”, the Soviet Union quickly became concerned. In hopes of quelling these liberating reform efforts, the Soviets reached out to their Warsaw Pact allies to plan an invasion into Czechoslovakia and intervene. Poland, Bulgaria, East Germany, and Hungary subsequently threw their support behind this Soviet plan - but two nations refused to do so. Both Albania and Romania were unwilling to join the invasion, and Romania’s Nicolae Ceaușescu went out of his way to condemn the operation. So what made Romania so unwilling to join the Warsaw Pac