The index's decrease compared with a target for a 0.3% drop.
The Johnson Redbook Sales Index also showed seasonally adjusted sales for the period improved 3.2% from a year earlier, compared with a target for an increase of 3.4%.
For the second week of April, sales rose 3.7% from a year earlier.
Redbook noted retailers benefited from a later Easter that boosted sales volume. Warmer weather also helped buoy customer traffic in many stores.
April is a four-week period on the retail calendar and ends May 3.