DM_GWPipeline_Gateway_MashupDataAccessError: We cannot convert the value null to type Text

Hi delaoda ,

Before spending more time debugging the model, it is worth stepping back and asking whether the problem lies in the model itself or in the architecture.

When a semantic model pulls directly from two different sources, especially when one of them goes through an on-premises gateway, you are exposing the model to all the variability those sources carry: changing schemas, timeouts, inconsistent type inference during mashup evaluation. The result is exactly what you are describing: an intermittent error that does not always appear and is hard to reproduce or diagnose.

The solution applied in modern data architectures is the Medallion pattern. Instead of having the model consume the sources directly, an ingestion process reads each source separately, transforms and consolidates the data into tables in a Gold layer within a Lakehouse. The semantic model then points exclusively to those tables.

With this approach, the model is completely decoupled from the sources. Type conversion issues, gateway dependencies, or any inconsistency coming from SAP WebI are handled once in the transformation layer, not on every scheduled refresh. You also gain independent control over the ingestion frequency of each source.

This is the same principle that has been applied in data engineering for years: separating the responsibility of acquiring the data from the responsibility of consuming it. Applying that principle to the semantic model eliminates at the root the class of errors you are seeing.

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Hi delaoda, 

Intermittent errors are always the worst to troubleshoot. 

Seeing as both your connections work independently, what I would recommend is keeping them seperate. 

Are you licenses for Fabric, or just Power BI? 

If you're licensed for Fabric, you can copy the PowerQuery code into a dataflow gen 2 and store the data in a lakehouse, you can then either use a third dataflow to combine them if you need them in one table, or you can keep them as seperate tables and pull them directly into your report. That would likely be the easiest way to achieve this. 

arabalca brought up medallion architecture, which is valuable, but it's not a one size fits all solution. So often do we see people make medallion layers that exist only to follow the bronze silver gold pattern, which I'm not sure is required here. 

If you're not licensed for Fabric, then you're in a bit of a pickle.

How is the data joined in your report? Are they kept as seperate tables? If so, you could make two "dummy" reports that each pull in the data from one source, and then in your real report connect to those semantic models.