diff --git a/api/api_storage.go b/api/api_storage.go
index 3e2eac3ca..e39acd034 100644
--- a/api/api_storage.go
+++ b/api/api_storage.go
@@ -355,12 +355,12 @@ type DealSchedule struct {
 type DagstoreShardInfo struct {
 	Key   string
 	State string
-	Error error
+	Error string
 }
 
 // DagstoreGCResult is the serialized form of dagstore.GCResult that we expose
 // through JSON-RPC to avoid clients having to depend on the dagstore lib.
 type DagstoreGCResult struct {
 	Key   string
-	Error error
+	Error string
 }
diff --git a/api/api_test.go b/api/api_test.go
index 738e1b067..e65d50ca3 100644
--- a/api/api_test.go
+++ b/api/api_test.go
@@ -76,7 +76,7 @@ func TestReturnTypes(t *testing.T) {
 						seen[typ] = struct{}{}
 
 						if typ.Kind() == reflect.Interface && typ != bareIface && !typ.Implements(jmarsh) {
-							t.Error("methods can't return interfaces", m.Name)
+							t.Error("methods can't return interfaces or struct types not implementing json.Marshaller", m.Name)
 						}
 
 						switch typ.Kind() {
diff --git a/api/docgen/docgen.go b/api/docgen/docgen.go
index 41968ef4a..8e16e2d42 100644
--- a/api/docgen/docgen.go
+++ b/api/docgen/docgen.go
@@ -278,12 +278,12 @@ func init() {
 	})
 	addExample(api.DagstoreGCResult{
 		Key:   "baga6ea4seaqecmtz7iak33dsfshi627abz4i4665dfuzr3qfs4bmad6dx3iigdq",
-		Error: nil,
+		Error: "<error>",
 	})
 	addExample(api.DagstoreShardInfo{
 		Key:   "baga6ea4seaqecmtz7iak33dsfshi627abz4i4665dfuzr3qfs4bmad6dx3iigdq",
 		State: "ShardStateAvailable",
-		Error: nil,
+		Error: "<error>",
 	})
 }
 
diff --git a/build/openrpc/miner.json.gz b/build/openrpc/miner.json.gz
index 2c0dbb082..266112d07 100644
Binary files a/build/openrpc/miner.json.gz and b/build/openrpc/miner.json.gz differ
diff --git a/cmd/lotus-miner/dagstore.go b/cmd/lotus-miner/dagstore.go
index 38247ce2e..52a52f2cb 100644
--- a/cmd/lotus-miner/dagstore.go
+++ b/cmd/lotus-miner/dagstore.go
@@ -118,7 +118,7 @@ var dagstoreGcCmd = &cli.Command{
 		}
 
 		for _, e := range collected {
-			if e.Error == nil {
+			if e.Error == "" {
 				_, _ = fmt.Fprintln(os.Stdout, e.Key, "success")
 			} else {
 				_, _ = fmt.Fprintln(os.Stdout, e.Key, "failed:", e.Error)
diff --git a/node/impl/storminer.go b/node/impl/storminer.go
index 6e222820a..b286b39f9 100644
--- a/node/impl/storminer.go
+++ b/node/impl/storminer.go
@@ -564,7 +564,12 @@ func (sm *StorageMinerAPI) DagstoreListShards(ctx context.Context) ([]api.Dagsto
 		ret = append(ret, api.DagstoreShardInfo{
 			Key:   k.String(),
 			State: i.ShardState.String(),
-			Error: i.Error,
+			Error: func() string {
+				if i.Error == nil {
+					return ""
+				}
+				return i.Error.Error()
+			}(),
 		})
 	}
 	return ret, nil
@@ -624,8 +629,13 @@ func (sm *StorageMinerAPI) DagstoreGC(ctx context.Context) ([]api.DagstoreGCResu
 	ret := make([]api.DagstoreGCResult, 0, len(res.Shards))
 	for k, err := range res.Shards {
 		ret = append(ret, api.DagstoreGCResult{
-			Key:   k.String(),
-			Error: err,
+			Key: k.String(),
+			Error: func() string {
+				if err == nil {
+					return ""
+				}
+				return err.Error()
+			}(),
 		})
 	}