Nested if returning false

I am helping a not for profit in creating input controls , The undermentione dofrmula returns correct results , with exception to the case where I15<0, where it is returning "FALSE" instead of returning the result as blank i.e"". I read other posts in the forum with answers to similar problem , but could not find where did i miss a "" ? Please help. =+IF(I15>0,IF(AND(F15="",G15=""),"Please input expense code and program code",IF(F15="","Please input expense code",IF(G15="","Please put program code",IF(B15="","Please input date",IF(C15="","Please specify receipt attached as yes or no",IF(D15="","Please specify vendor name","")))))))