Vanguard Group’s Energy Fund was temporarily closed for new shareholders to protect the existing ones. The fund that currently has $5.3 billion in assets was closed for a "cooling-off" period, the company announced Wednesday.

"Given the intense interest in the energy sector as a whole and the level of cash flow into Vanguard Energy Fund, the board of trustees decided that this was a prudent step to protect existing shareholders," said Vanguard Chairman Jack Brennan.

"Vanguard funds are designed for long-term investors, and we are concerned that some investors may be gravitating toward the Energy Fund based on its recent short-term performance record."

The Energy Fund was a top performer in the Vanguard Group, and the seventh in Vanguard to close close in the past 30 months.