This closes #1677, fix the incorrect custom number format ID allocated

- Improve compatibility with empty custom number format code

styles.go

@@ -1694,20 +1694,18 @@ func newNumFmt(styleSheet *xlsxStyleSheet, style *Style) int {
 
 // setCustomNumFmt provides a function to set custom number format code.
 func setCustomNumFmt(styleSheet *xlsxStyleSheet, style *Style) int {
-	nf := xlsxNumFmt{FormatCode: *style.CustomNumFmt}
+	nf := xlsxNumFmt{NumFmtID: 163, FormatCode: *style.CustomNumFmt}
-
+	if styleSheet.NumFmts == nil {
-	if styleSheet.NumFmts != nil {
+		styleSheet.NumFmts = &xlsxNumFmts{}
-		nf.NumFmtID = styleSheet.NumFmts.NumFmt[len(styleSheet.NumFmts.NumFmt)-1].NumFmtID + 1
-		styleSheet.NumFmts.NumFmt = append(styleSheet.NumFmts.NumFmt, &nf)
-		styleSheet.NumFmts.Count++
-	} else {
-		nf.NumFmtID = 164
-		numFmts := xlsxNumFmts{
-			NumFmt: []*xlsxNumFmt{&nf},
-			Count:  1,
-		}
-		styleSheet.NumFmts = &numFmts
 	}
+	for _, numFmt := range styleSheet.NumFmts.NumFmt {
+		if numFmt != nil && nf.NumFmtID < numFmt.NumFmtID {
+			nf.NumFmtID = numFmt.NumFmtID
+		}
+	}
+	nf.NumFmtID++
+	styleSheet.NumFmts.NumFmt = append(styleSheet.NumFmts.NumFmt, &nf)
+	styleSheet.NumFmts.Count = len(styleSheet.NumFmts.NumFmt)
 	return nf.NumFmtID
 }
 

xmlStyles.go

@@ -296,7 +296,7 @@ type xlsxNumFmts struct {
 // of a cell.
 type xlsxNumFmt struct {
 	NumFmtID     int    `xml:"numFmtId,attr"`
-	FormatCode   string `xml:"formatCode,attr,omitempty"`
+	FormatCode   string `xml:"formatCode,attr"`
 	FormatCode16 string `xml:"http://schemas.microsoft.com/office/spreadsheetml/2015/02/main formatCode16,attr,omitempty"`
 }