Dual Unit of Measure - P4210

We have items set up as dual unit of measure  - primary = EA and secondary = LB.  The work order production for this item captured the Eaches and Weights uniquely by lot# and without error.  When a sales order is attempted to ship a lot, I get a Unit of Measure Invalid for Item Error.  I attempted to add a conversion for eaches to pounds - that did not generate the error, but it not relieve inventory correctly because the weight did not match the conversion weight.  Any ideas?