Many investors consider bonds the "safe" investment. But in considering a bond "safe" you may be failing to manage some very real risks that bond investing presents. These risks are why bonds, like equities, require ongoing oversight and active management.
You have three risks with an individual bond. The first is the risk of default. The second is the risk that you will need your principal prior to maturity and be forced to sell a bond at a loss. The third risk is that when your bond matures, you will be unable to replace it with another bond paying comparable interest.
Unless the bond fund is a Unit Investment Trust (UIT,) it has no maturity and thus no obligation to return your principal. If a bond fund falls in value, there is no option of simply holding it until maturity to recapture your principal. The value of your investment in a bond fund will change in response market conditions and interest rates. On the other hand, you gain liquidity through the ability to sell virtually all bond funds at the current fund value (NAV).