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

[kirk]

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 SchmidtSent 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)
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."
For information, I paste below the overview of the algorithm D :


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. Thanks so much for your valuable help.
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