problem in makig a field obligatory

hi all

i am using a statement

s_cust for table1-field1 obligatory.

but in the selection screen the obligatory is being done for first range only

ie the low value ,the high value if user dont fill then also the program executes

i want to make both the ranges as obligatory so that user need to specify the range for the field1 and need to fill in both the ranges (low and high but presently only with this statement only first low is obligatory high value if we remain blank the program executes

pls suggest