Why Blank Slicer Values Hide Fact Rows in Power BI

Introduction

Power BI users often encounter a puzzling scenario: a slicer shows a blank value, but selecting it doesn’t reveal the expected rows in a table visual, unless a measure is added. This behaviour is especially common in models with a fact table and a related dimension table. Let’s unpack why this happens and how to fix it.

Let's dig into the scenario and the setup.

The Setup

Imagine a Power BI model with two tables:

• Fact_students: Contains student data, including a school_district column.
• Dim_District: A dimension table with valid school_district values.

There’s a relationship from Dim_District[school_district] to Fact_students[school_district], typically single-direction and many-to-one.

The Problem

To demonstrate the issue, school_district for ID 1 in the fact table has been marked as null

Created a measure to show the Max GPA for a student and in a table visual included the student ID, the GPA measure.

As you can see from the above visual Fact_students, ID 1 have a blank school_district. Even though Dim_District has no blank values, the slicer on school_district shows a blank option. Selecting this blank in the slicer doesn’t show the corresponding fact rows in a table visual unless you add a measure like row count.

Why This Happens

Power BI slicers filter visuals using the dimension table. When you select blank in the slicer, Power BI applies a filter like:

TREATAS({BLANK()}, 'Dim_District'[school_district])

This filter only includes fact rows that match a blank value in the dimension. But if Dim_District has no blank row, the blank fact row has no match. As a result:

• The row is excluded from the visual.
• It appears only when a measure is added, because measures evaluate over all rows, even those without matching dimension keys.

The DAX Query Confirms It

The DAX query from the visual might look like:

SUMMARIZECOLUMNS(

    ROLLUPADDISSUBTOTAL('Fact_students'[student_id], "IsGrandTotalRowTotal"),

    TREATAS({BLANK()}, 'Dim_District'[school_district]),

    "GPA_M", 'Fact_students'[GPA_M]

)

This query filters using the dimension, and rows without a matching key are dropped.

How to Fix It

In this instance I created a row count measure and included in the visual this overridden the default filter context and gave me the null value

If this method is not looking good There are several ways to resolve this:

1. Add a Blank Row to Dim_District

Include a row with a blank school_district value. This allows the blank fact row to match and be included in visuals.

2. Replace Blanks with a Default Value

Use Power Query or DAX to replace blank keys with a default like "Unknown" and add a matching row in the dimension.

3. Use Bi-Directional Filtering

Enable bidirectional filtering on the relationship. This allows the slicer to filter the fact table directly, but use with caution.

4. Use Measure-Based Logic

Design visuals around measures that include all rows, even those without matching dimension keys.

Conclusion

Blank slicer values in Power BI can be misleading. They appear because of unmatched fact rows, but don’t filter as expected unless the model supports it. Understanding how relationships and filter context work is key to building reliable, intuitive reports