LykosAI/StabilityMatrix · error · ValidationException

Sampler not selected

Error message

Sampler not selected

What it means

SamplerCardViewModel.ApplyStepsInitialCustomSampler throws this ValidationException when applying a custom-sampler step: SelectedSampler is null, so Connections.PrimarySampler cannot be set. The initial custom sampler step must define which ComfyUI sampler (e.g. euler, dpmpp_2m) to use, so it fails fast.

Solutions

1. Choose a sampler in the sampler card's sampler dropdown before generating.
2. Programmatically set vm.SelectedSampler to a valid ComfySampler value.
3. Restore default sampler settings for the card to repopulate the selection.
4. Guard generation: if SelectedSampler is null, surface a friendly prompt to pick one instead of letting the build throw.

Example fix

// before
var samplerCard = new SamplerCardViewModel();
await samplerCard.ApplyStepsInitialCustomSampler(stepArgs); // throws
// after
var samplerCard = new SamplerCardViewModel();
samplerCard.SelectedSampler = ComfySampler.EulerAncestral;
samplerCard.SelectedScheduler = ComfyScheduler.Karras;
await samplerCard.ApplyStepsInitialCustomSampler(stepArgs);

if (samplerCard.SelectedSampler is null)
    throw new InvalidOperationException("Sampler must be selected before generation");

var samplerReady = samplerCard.SelectedSampler is ComfySampler;

try { await samplerCard.ApplyStepsInitialCustomSampler(args); }
catch (ValidationException ex) when (ex.Message == "Sampler not selected")
{
    // prompt user to choose a sampler
}

When it happens

Trigger: Running the workflow build path that calls ApplyStepsInitialCustomSampler with the sampler card's SelectedSampler property never set (no sampler chosen in the sampler card).

Common situations: User clears the sampler dropdown in the sampler card; a sampler pack/custom sampler selection failed to load so the default is null; programmatically constructing a SamplerCardViewModel without assigning SelectedSampler.

Understand the failure class

Background: "Must pass :limit option" / "Missing required option" — required option errors explained — this error's family across 41 libraries.

Related errors

• Scheduler not selected
• Length cannot be null when latentType is Hunyuan
• Prompt extensions not installed
• No ImageSource
• Model not selected

AI-assisted analysis of LykosAI/StabilityMatrix@af93d6ef57 (2026-09-12). Data as JSON: /api/errors/437d7c0717db8a98. Report an issue: GitHub.

Appendix: source

Thrown at StabilityMatrix.Avalonia/ViewModels/Inference/SamplerCardViewModel.cs:350

        ModuleApplyStepEventArgs e,
        bool useFluxGuidance,
        bool useFlux2Scheduler = false
    )
    {
        // Provide temp values
        e.Temp = e.CreateTempFromBuilder();

        // Apply steps from our addons
        ApplyAddonSteps(e);

        // Get primary as latent using vae
        var primaryLatent = e.Builder.GetPrimaryAsLatent(
            e.Temp.Primary!.Unwrap(),
            e.Builder.Connections.GetDefaultVAE()
        );

        // Set primary sampler and scheduler
        var primarySampler = SelectedSampler ?? throw new ValidationException("Sampler not selected");
        e.Builder.Connections.PrimarySampler = primarySampler;

        var primaryScheduler = SelectedScheduler ?? throw new ValidationException("Scheduler not selected");
        e.Builder.Connections.PrimaryScheduler = primaryScheduler;

        // for later inheritance if needed
        e.Builder.Connections.PrimaryCfg = CfgScale;
        e.Builder.Connections.PrimarySteps = Steps;
        e.Builder.Connections.PrimaryModelType = SelectedModelType;

        // KSamplerSelect
        var kSamplerSelect = e.Nodes.AddTypedNode(
            new ComfyNodeBuilder.KSamplerSelect
            {
                Name = e.Nodes.GetUniqueName(nameof(ComfyNodeBuilder.KSamplerSelect)),
                SamplerName = e.Builder.Connections.PrimarySampler?.Name!,
            }
        );

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