qax-os/excelize · error

custom number format can not be empty

Error message

custom number format can not be empty

What it means

ErrCustomNumFmt is returned when a Style specifies CustomNumFmt as a non-nil pointer to an empty string. The library treats a present-but-empty custom number format as invalid, since an empty format code cannot be applied to cells. It is raised in parseFormatStyleSet (styles.go) and surfaced via NewStyle/NewConditionalStyle.

Source

Thrown at errors.go:46

	// characters length that exceeds the limit.
	ErrCellCharsLength = fmt.Errorf("cell value must be 0-%d characters", TotalCellChars)
	// ErrCellStyles defined the error message on cell styles exceeds the limit.
	ErrCellStyles = fmt.Errorf("the cell styles exceeds the %d limit", MaxCellStyles)
	// ErrChartTitle defined the error message on both formula and rich text for
	// chart title.
	ErrChartTitle = errors.New("cannot set both 'Formula' and 'Paragraph' for chart title")
	// ErrColumnNumber defined the error message on receive an invalid column
	// number.
	ErrColumnNumber = fmt.Errorf("the column number must be greater than or equal to %d and less than or equal to %d", MinColumns, MaxColumns)
	// ErrColumnWidth defined the error message on receive an invalid column
	// width.
	ErrColumnWidth = fmt.Errorf("the width of the column must be less than or equal to %d characters", MaxColumnWidth)
	// ErrCoordinates defined the error message on invalid coordinates tuples
	// length.
	ErrCoordinates = errors.New("coordinates length must be 4")
	// ErrCustomNumFmt defined the error message on receive the empty custom
	// number format.
	ErrCustomNumFmt = errors.New("custom number format can not be empty")
	// ErrDataValidationFormulaLength defined the error message for receiving a
	// data validation formula length that exceeds the limit.
	ErrDataValidationFormulaLength = fmt.Errorf("data validation must be 0-%d characters", MaxFieldLength)
	// ErrDataValidationRange defined the error message on set decimal range
	// exceeds limit.
	ErrDataValidationRange = errors.New("data validation range exceeds limit")
	// ErrDefinedNameDuplicate defined the error message on the same name
	// already exists on the scope.
	ErrDefinedNameDuplicate = errors.New("the same name already exists on the scope")
	// ErrDefinedNameScope defined the error message on not found defined name
	// in the given scope.
	ErrDefinedNameScope = errors.New("no defined name on the scope")
	// ErrExistsSheet defined the error message on given sheet already exists.
	ErrExistsSheet = errors.New("the same name sheet already exists")
	// ErrExistsTableName defined the error message on given table already
	// exists.
	ErrExistsTableName = errors.New("the same name table already exists")
	// ErrFillType defined the error message on receive an invalid fill type.

View on GitHub (pinned to f2483381fb)

Solutions

1. Set the CustomNumFmt string to a valid format code (e.g. "0.00", "yyyy-mm-dd") before calling NewStyle/NewConditionalStyle
2. If no custom format is needed, leave CustomNumFmt nil instead of pointing at an empty string
3. Validate the format string non-empty before constructing the Style
4. Check where the value is sourced (config, template) to fix the upstream empty value

Example fix

// before
var exp string
_, err := f.NewConditionalStyle(&Style{CustomNumFmt: &exp}) // ErrCustomNumFmt
// after
numFmt := "0.00"
_, err := f.NewConditionalStyle(&Style{CustomNumFmt: &numFmt})

func validCustomNumFmt(s *string) bool { return s == nil || *s != "" }

When it happens

Trigger: Calling f.NewConditionalStyle(&Style{CustomNumFmt: &empty}) or f.NewStyle with CustomNumFmt pointing to "" — the pointer is non-nil so the field is 'set', but the string is empty.

Common situations: Declaring var fmt string (zero value "") and taking its address; template/config values that failed to populate leaving an empty string; copying a Style struct where the original had a valid format that got cleared.

Understand the failure class

Background: "must not be empty", "cannot be empty" — required-field validation errors across open-source libraries — this error's family across 41 libraries.

Related errors

• ErrUnsupportedNumberFormat

AI-assisted analysis of qax-os/excelize@f2483381fb (2026-09-02). Data as JSON: /api/errors/83878652b33845d8. Report an issue: GitHub.