The last time I wrote about savings bonds, in May, plenty of readers thought my math had to be wrong because I reported the “earnings rate” on the Series I bond would be zero percent.

But no. Had you bought a Series I bond between May 1 and Sept. 30, the Treasury Department would pay you nothing for six months after the issue date. It was equivalent to sticking $50 in a shoe box for half a year.

Zero percent came as quite a shock for some, because in the previous six months they could have bought an I bond paying 5.64 percent.

Sales of savings bonds have fallen steadily. According to the Bureau of Public Debt, sales of Series I bonds went from $180 million in April, before the rate change, to a low of $52 million in September.

For the federal fiscal year ended Sept. 30, the U.S. government sold $1.66 billion in Series I bonds, down from $1.92 billion the previous year. An all-time high of $8.12 billion in Series I bonds were sold during the 2003 fiscal year.

As an investment, savings bonds tend to appeal more to the Greatest Generation than Generation Wired. The readers who called me had different reasons for buying them, but they had a common question: “Will the rate ever go back up?”

It was a good one because Series I bonds have a rate that adjusts with inflation. Given that more economists are worried about deflation right now, that didn’t bode well for a rebound in savings-bond rates.

But Monday, the Treasury Department set the rate at 3.36 percent for a Series I bond bought between Nov. 1 and April 30, 2010.

What accounted for the shift? First, Treasury raised the fixed portion of the I-bond rate from 0.1 percent to 0.3 percent. But also, the Consumer Price Index for all urban consumers rose from 212.709 to 215.969 from March 2009 through September 2009, according to the Bureau of Labor Statistics, which tracks inflation.

For the previous six months, the CPI was negative, but you can’t have a negative earnings rate on a savings bond. So Treasury set the Series I rate at zero for bonds bought between May 1 and Oct. 31.

Yesterday, Treasury also increased the earnings rate on Series EE bonds to 1.2 percent from 0.7 percent.