[Qgis-user] QGIS modeler error message : "Unable to evaluate the expression for the extra parameter : no root node!"

[Emma Hain]

Also maybe use the `Run Selected Steps` option to check your outputs as you
go, if you aren't already doing that.

Kind regards
Em


On Wed, 14 Jan 2026 at 22:10, kirk via QGIS-User <qgis-user at lists.osgeo.org>
wrote:

> Good Morning.
>
> A quick error check is to write the output of each function so that you
> can confirm all is working as it should.
>
> Your error coukd be a faulty input into the algorithm D, in your case the
> ressult of A or B or the vector input..
>
> Kirk Schmidt
>
>
>
>
>
> Sent from my Galaxy
>
>
> -------- Original message --------
> From: celati Laurent via QGIS-User <qgis-user at lists.osgeo.org>
> Date: 2026-01-14 5:28 a.m. (GMT-04:00)
> To: qgis-user <qgis-user at lists.osgeo.org>
> Subject: [Qgis-user] QGIS modeler error message : "Unable to evaluate the
> expression for the extra parameter : no root node!"
>
> Dear all,
>
> I'm working with QGIS 3.4. This is my first QGIS model. I'm facing to an
> error message during the running of the modeler. If I understand the error
> message, the error seems to occur during the step "D" on my little
> screenshot (below) showing the modeler. For information, I've defined a
> conditional branch. In addition to the screenshot of the modeler, I paste
> also the error message.
>
> Modeler explanation :
>
>    - The output of the algorithm A is a raster clipped with a polygon (an
>    area of interest)
>    - For the algorithm B (difference vector processing between 2 input
>    layers), I've setted a dependency so that it only triggers if a condition
>    is met (we can see the conditional branch 'C' on the screenshot).
>    - Algorithm D (clip raster with a polygon) takes as input the raster
>    output/result of the algorithm A. And as a mask layer, the vector result of
>    algorithm B (difference). For this algorithm D, I've still enable the same
>    dependency setted for the algorithm B.
>    - For algorithm E (raster polygonization),we can see there are 2
>    possible raster inputs. The output of the algorithm A and the output of the
>    algorithm D. Thus, as input to algorithm E, I set the following expression
>    (pre-calculated value):
>
>  if (@exclude_forest_zones , @cut_raster_segmentation_by_mask_vector_NO_foret_OUTPUT , @cut_raster_segmentation_by_study_area_vector_OUTPUT)
>
>
> [image: image.png]
>
>
> When I run the modeler, I get an error message that seems to be related to
> algorithm D. Below is the error message:
>
> "Unable to evaluate the expression for the extra parameter to clip the raster segmentation by the NO forest mask: no root node! Execution failed after 8 seconds."
>
>
> [image: image.png]
> For information, I paste below the overview of the algorithm D :
>
> [image: image.png]
>
> And I paste below for information the overview of the algorithm E : As you
> can see on the screenshot, I haven't defined any dependencies for algorithm
> this algo E.
>
> [image: image.png]
>
> Thanks so much for your valuable help.
>
>
>
>
>
>
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