diff --git a/checker/provider.go b/checker/provider.go
index ad7e3b3..a7d9825 100644
--- a/checker/provider.go
+++ b/checker/provider.go
@@ -4,10 +4,16 @@ package checker
 
 import (
 	"context"
+	"errors"
 
 	sdk "git.happydns.org/checker-sdk-go/checker"
 )
 
+// ErrNoCollector is returned by Collect when the provider was constructed
+// without a CollectFn (e.g. when registered host-side as an externalizable-only
+// provider). Callers should route the observation to an external checker.
+var ErrNoCollector = errors.New("dnsviz: provider has no local collector; use an external checker")
+
 // CollectFn is the function signature for the DNSViz data collection step.
 // The checker package is decoupled from the subprocess invocation so it can
 // be imported without GPL obligations. Implementations live in the binary or
@@ -27,5 +33,8 @@ func (p *dnsvizProvider) Key() sdk.ObservationKey {
 }
 
 func (p *dnsvizProvider) Collect(ctx context.Context, opts sdk.CheckerOptions) (any, error) {
+	if p.collect == nil {
+		return nil, ErrNoCollector
+	}
 	return p.collect(ctx, opts)
 }