REBOL 3 Errors: wrong-denom

A math operation was performed on money values of two different denominations. For instance, when trying to add USD$1.00 to DEN$1.50.

Message:

[:arg1 "not same denomination as" :arg2]

Example:

filter-error try [US$1.50 + DM$1.50]
** Script Error: US$1.50 not same denomination as DM$1.50
** Where: US$1.50 + DM$1.50