dotnet/machinelearning · error · ArgumentException

Strings.MismatchedValueType (formatted with typeof(string))

Error message

Strings.MismatchedValueType (formatted with typeof(string))

What it means

Thrown by ArrowStringDataFrameColumn.FillNullsImplementation when the object passed to FillNulls is not a string. The column stores UTF-8 byte buffers of strings, so only a string replacement value is meaningful; any other runtime type (int, byte[], etc.) triggers this guard, whose message is formatted with typeof(string) to tell the caller the only accepted type.

Source

Thrown at src/Microsoft.Data.Analysis/DataFrameColumns/ArrowStringDataFrameColumn.cs:554

            }

            ArrowStringDataFrameColumn ret = new ArrowStringDataFrameColumn(Name);
            for (long i = 0; i < Length; i++)
            {
                ret.Append(IsValid(i) ? GetBytes(i) : Encoding.UTF8.GetBytes(value));
            }
            return ret;
        }

        protected override DataFrameColumn FillNullsImplementation(object value, bool inPlace)
        {
            if (value is string valueString)
            {
                return FillNulls(valueString, inPlace);
            }
            else
            {
                throw new ArgumentException(String.Format(Strings.MismatchedValueType, typeof(string)), nameof(value));
            }
        }

        /// <inheritdoc/>
        public new ArrowStringDataFrameColumn DropNulls()
        {
            return (ArrowStringDataFrameColumn)DropNullsImplementation();
        }

        protected override DataFrameColumn DropNullsImplementation()
        {
            var ret = new ArrowStringDataFrameColumn(Name);

            for (long i = 0; i < Length; i++)
            {
                if (IsValid(i))
                    ret.Append(GetBytes(i));
            }

View on GitHub (pinned to 7b76e69cf9)

Solutions

1. Pass a string value to DataFrame.FillNulls, e.g. col.FillNulls("N/A") or the in-place overload FillNulls(value, inPlace: true)
2. If the fill value arrives as object from generic code, check `value is string` before calling and convert or reject it upstream
3. To fill with a non-string value, first convert the column to a matching primitive column type (e.g. via ConvertTo or ChangeColumnType) and fill there

When it happens

Trigger: Thrown at src/Microsoft.Data.Analysis/DataFrameColumns/ArrowStringDataFrameColumn.cs:554 when the library encounters an invalid state.

Common situations: See trigger scenarios.

AI-assisted analysis of dotnet/machinelearning@7b76e69cf9 (2026-09-11). Data as JSON: /api/errors/de4ac4fbd8d9e8cb. Report an issue: GitHub.