start-of-selection being triggered on Enter

I am having a problem with the start-of-selection being triggered with the enter key after I edited the selections and change a value.  Specifically, I have 3 fields on the selection screen.  I  enter the value for the first field and hit Execute.  This triggers AT SELECTION-SCREEN and calls my subroutine to edit the fields.  Fields 2 and 3 are blank.  The edit code will give an error if either is blank.  So, if I enter a value in the 3rd field and press enter, sy-ucomm is blank and the start-of-selection is hit and the program executes. If I enter any or all of the fields and hit enter without first hitting execute, the selection screen remains and the program does call the edit subroutine and the program does not start.  After I hit execute, it will call the edit routine  from within the At Selection-Screen and the program will execute