You SHOULD replace the Nz with an IIf(IsNull()) construct as it's prone to Errors for NOT reason.

See documentation for IIf and IsNull

When put together:Nz(expr, [valueifnull])
becomesIIf(IsNull(expr), valueifnull, valueifnotnull)

Examples
Default: Nz(tbl.A) => IIf(IsNull(tbl.A), '', tbl.A)
With fallback: Nz(tbl.A, tbl.B) => IIf(IsNull(tbl.A), tbl.B, tbl.A)