diff --git a/openapi.go b/openapi.go
index 5406142..5c28a29 100644
--- a/openapi.go
+++ b/openapi.go
@@ -63,6 +63,10 @@ const openAPIDialect = "https://spec.openapis.org/oas/3.1/dialect/base"
 //
 //	data, err := (&api.SpecBuilder{Title: "Service", Version: "1.0.0"}).Build(engine.Groups())
 func (sb *SpecBuilder) Build(groups []RouteGroup) ([]byte, error) {
+	if sb == nil {
+		sb = &SpecBuilder{}
+	}
+
 	prepared := prepareRouteGroups(groups)
 
 	spec := map[string]any{
@@ -214,6 +218,10 @@ func (sb *SpecBuilder) Build(groups []RouteGroup) ([]byte, error) {
 //
 //	data, err := (&api.SpecBuilder{Title: "Service"}).BuildIter(api.RegisteredSpecGroupsIter())
 func (sb *SpecBuilder) BuildIter(groups iter.Seq[RouteGroup]) ([]byte, error) {
+	if sb == nil {
+		sb = &SpecBuilder{}
+	}
+
 	return sb.Build(collectRouteGroups(groups))
 }
 
diff --git a/openapi_test.go b/openapi_test.go
index 56de063..678e8d5 100644
--- a/openapi_test.go
+++ b/openapi_test.go
@@ -208,6 +208,32 @@ func TestSpecBuilder_Good_EmptyGroups(t *testing.T) {
 	}
 }
 
+func TestSpecBuilder_Good_NilReceiverIsZeroValueSafe(t *testing.T) {
+	var sb *api.SpecBuilder
+
+	data, err := sb.Build(nil)
+	if err != nil {
+		t.Fatalf("unexpected error: %v", err)
+	}
+
+	var spec map[string]any
+	if err := json.Unmarshal(data, &spec); err != nil {
+		t.Fatalf("invalid JSON: %v", err)
+	}
+
+	if spec["openapi"] != "3.1.0" {
+		t.Fatalf("expected openapi=3.1.0, got %v", spec["openapi"])
+	}
+
+	paths, ok := spec["paths"].(map[string]any)
+	if !ok {
+		t.Fatalf("expected paths object, got %T", spec["paths"])
+	}
+	if _, ok := paths["/health"]; !ok {
+		t.Fatal("expected /health path to be present")
+	}
+}
+
 func TestSpecBuilder_Good_CustomSecuritySchemesAreMerged(t *testing.T) {
 	sb := &api.SpecBuilder{
 		Title:   "Test",