I have two formulas, this one works:

=SOMMEPROD((factory=$A$3)*(type=B$5)*(year=$H$1)*( month=$J$1)*(waste_t))

This one doesn't, it gives me the #value error:

=SOMMEPROD((factory=$A$3)*(type=B$5)*(year=$H$1)*( month=$J$1)*($L7))

L7 is a cell with "waste_t" written in it