dotnet/efcore · error · InvalidOperationException

An error occurred while reading a database value. The…

Error message

An error occurred while reading a database value. The expected type was '{expectedType}' but the actual value was 'null'.

What it means

Thrown by BufferedDataReader.ThrowReadValueException when a non-nullable property/column receives a null (DBNull or NullReferenceException) value from the database during materialization. The message names the expected CLR type to make the null-vs-non-null mismatch obvious.

Solutions

Make the CLR property nullable (int? / Nullable<T>) to match the database column's nullability.
Update the database schema to make the column NOT NULL (and backfill existing NULLs).
If using a database view, ensure the view never returns NULL for that column.
Configure the property as .IsRequired() only when the column truly cannot be null.

Example fix

// before
public class User
{
    public int Id { get; set; }
    public int Age { get; set; } // column Age is NULL-able in DB
}

// after
public class User
{
    public int Id { get; set; }
    public int? Age { get; set; }
}

Defensive patterns

Strategy: validation

Validate before calling

static void EnsureNullableClrForNullableColumn(IProperty property, bool columnIsNullable)
{
    if (columnIsNullable && !property.IsNullable)
        throw new InvalidOperationException($"Property {property.Name} is non-nullable but its column is nullable; NULL reads will fail.");
}

Type guard

static bool IsClrNullable(Type t) => !t.IsValueType || Nullable.GetUnderlyingType(t) is not null;

Prevention

Match CLR nullability to column nullability; use int? for nullable DB columns.
After schema changes, re-scaffold or update model to reflect column nullability.
Use .IsRequired() only when the column is truly NOT NULL.

When it happens

Trigger: A column mapped to a non-nullable CLR type (int, bool, Guid, etc.) contains NULL in the database. ThrowReadValueException catches the NullReferenceException from GetFieldValue and reports ErrorMaterializingValueNullReference.

Common situations: Database column allows NULL but entity property is non-nullable (int instead of int?). Stale schema vs model (column added as nullable, then property typed non-nullable). Default value not set on a new NOT NULL column during migration. View/procedure returning NULL for a not-null field.

Related errors

AI-assisted analysis of dotnet/efcore@3a2006ef56 (2026-08-11). Data as JSON: /api/errors/9dd12f4f58bb0760. Report an issue: GitHub.

Appendix: source

Thrown at src/EFCore.Relational/Query/Internal/BufferedDataReader.cs:1934

                message = exception is NullReferenceException
                    || Equals(value, DBNull.Value)
                        ? RelationalStrings.ErrorMaterializingPropertyNullReference(entityType, propertyName, expectedType)
                        : exception is InvalidCastException
                            ? CoreStrings.ErrorMaterializingPropertyInvalidCast(entityType, propertyName, expectedType, actualType)
                            : RelationalStrings.ErrorMaterializingProperty(entityType, propertyName);
            }
            else
            {
                message = exception is NullReferenceException
                    || Equals(value, DBNull.Value)
                        ? RelationalStrings.ErrorMaterializingValueNullReference(expectedType)
                        : exception is InvalidCastException
                            ? RelationalStrings.ErrorMaterializingValueInvalidCast(expectedType, actualType)
                            : RelationalStrings.ErrorMaterializingValue;
            }

            throw new InvalidOperationException(message, exception);
        }
    }
}

View on GitHub (pinned to 3a2006ef56)