Sumproduct gives Value error

[tabkaz]

I have a data on multiple sheets, I am using Sumproduct formula for summation of values matching date and time, later on, a product of a time duration and finally a division by 60. But when there is any blank cell for multiplication it gives value error.
Also, want to compact this formula as data are in multiple 5 sheets.

=SUMPRODUCT(('BR1 grid'!$A$7:$A$54=$A3)*('BR1 grid'!$C$5:$AG$5=B$2),'BR1 grid'!$C$7:$AG$54)*'BR1 grid'!$L$1/60+SUMPRODUCT(('BR2 grid'!$A$7:$A$54=$A3)*('BR2 grid'!$C$5:$AG$5=B$2),'BR2 grid'!$C$7:$AG$54)*'BR2 grid'!$L$1/60

See sheet 4 yellow shaded