diff --git a/pkg/actor/mutation_registry.go b/pkg/actor/mutation_registry.go
index 970944d..1e5f66a 100644
--- a/pkg/actor/mutation_registry.go
+++ b/pkg/actor/mutation_registry.go
@@ -12,7 +12,7 @@ import (
 type ApplyResult struct {
 	Type     string        `json:"type"`
 	Mutation proto.Message `json:"mutation"`
-	Error    error         `json:"error,omitempty"`
+	Error    string        `json:"error,omitempty"`
 }
 
 type MutationRegistry interface {
@@ -167,11 +167,11 @@ func (r *ProtoMutationRegistry) Apply(grain any, msg ...proto.Message) ([]ApplyR
 		entry, ok := r.mutationRegistry[rt]
 		r.mutationRegistryMu.RUnlock()
 		if !ok {
-			results = append(results, ApplyResult{Error: ErrMutationNotRegistered, Type: rt.Name(), Mutation: m})
+			results = append(results, ApplyResult{Error: ErrMutationNotRegistered.Error(), Type: rt.Name(), Mutation: m})
 			continue
 		}
 		err := entry.Handle(grain, m)
-		results = append(results, ApplyResult{Error: err, Type: rt.Name(), Mutation: m})
+		results = append(results, ApplyResult{Error: err.Error(), Type: rt.Name(), Mutation: m})
 	}
 
 	// if entry.updateTotals {