diff --git a/generate/generate.go b/generate/generate.go
index 019ca88..4c91872 100644
--- a/generate/generate.go
+++ b/generate/generate.go
@@ -12,6 +12,9 @@ import (
 	"github.com/vektah/gqlparser/v2/formatter"
 )
 
+// Set to true to test features that aren't yet really ready.
+var allowBrokenFeatures = false
+
 var fileTemplate = mustTemplate("operation.go.tmpl")
 
 // generator is the context for the codegen process (and ends up getting passed
@@ -168,6 +171,10 @@ func Generate(config *Config) (map[string][]byte, error) {
 		return nil, fmt.Errorf("no queries found in %v", config.Operations)
 	}
 
+	if len(document.Fragments) > 0 && !allowBrokenFeatures {
+		return nil, fmt.Errorf("genqlient does not yet support fragments")
+	}
+
 	g := newGenerator(config, schema)
 	for _, op := range document.Operations {
 		if err = g.addOperation(op); err != nil {
diff --git a/generate/generate_test.go b/generate/generate_test.go
index d1ded8c..fa4bfeb 100644
--- a/generate/generate_test.go
+++ b/generate/generate_test.go
@@ -48,6 +48,8 @@ func gofmt(filename, src string) (string, error) {
 // snapshots don't even get compiled!
 func TestGenerate(t *testing.T) {
 	update := (os.Getenv("UPDATE_SNAPSHOTS") == "1")
+	// we can test parts of features even if they're not done yet!
+	allowBrokenFeatures = true
 
 	files, err := ioutil.ReadDir(dataDir)
 	if err != nil {
diff --git a/generate/types.go b/generate/types.go
index d0c756e..dfb110e 100644
--- a/generate/types.go
+++ b/generate/types.go
@@ -269,6 +269,10 @@ func (builder *typeBuilder) writeTypedef(typedef *ast.Definition, fields []field
 		return builder.maybeWriteUnmarshal(fields)
 
 	case ast.Interface, ast.Union:
+		if !allowBrokenFeatures {
+			return fmt.Errorf("not implemented: %v", typedef.Kind)
+		}
+
 		// First, write the interface type.
 		builder.WriteString("interface {\n")
 		implementsMethodName := fmt.Sprintf("implementsGraphQLInterface%v", builder.typeName)