diff --git a/bits/67_wsxml.js b/bits/67_wsxml.js
index c8a7f99..d8cc812 100644
--- a/bits/67_wsxml.js
+++ b/bits/67_wsxml.js
@@ -242,7 +242,7 @@ function write_ws_xml_sheetviews(ws, opts, idx, wb)/*:string*/ {
 }
 
 function write_ws_xml_cell(cell/*:Cell*/, ref, ws, opts/*::, idx, wb*/)/*:string*/ {
-	if(cell.v === undefined && cell.f === undefined || cell.t === 'z') return "";
+	if(cell.v === undefined && typeof cell.f !== "string" || cell.t === 'z') return "";
 	var vv = "";
 	var oldt = cell.t, oldv = cell.v;
 	if(cell.t !== "z") switch(cell.t) {
@@ -278,7 +278,7 @@ function write_ws_xml_cell(cell/*:Cell*/, ref, ws, opts/*::, idx, wb*/)/*:string
 			o.t = "str"; break;
 	}
 	if(cell.t != oldt) { cell.t = oldt; cell.v = oldv; }
-	if(cell.f) {
+	if(typeof cell.f == "string" && cell.f) {
 		var ff = cell.F && cell.F.slice(0, ref.length) == ref ? {t:"array", ref:cell.F} : null;
 		v = writextag('f', escapexml(cell.f), ff) + (cell.v != null ? v : "");
 	}