diff --git a/statefun-sdk-go/v3/pkg/statefun/handler.go b/statefun-sdk-go/v3/pkg/statefun/handler.go
index 410955f5c..77f64227f 100644
--- a/statefun-sdk-go/v3/pkg/statefun/handler.go
+++ b/statefun-sdk-go/v3/pkg/statefun/handler.go
@@ -24,8 +24,9 @@ import (
 	"net/http"
 	"sync"
 
-	"github.com/apache/flink-statefun/statefun-sdk-go/v3/pkg/statefun/internal/protocol"
 	"google.golang.org/protobuf/proto"
+
+	"github.com/apache/flink-statefun/statefun-sdk-go/v3/pkg/statefun/internal/protocol"
 )
 
 // StatefulFunctions is a registry for multiple StatefulFunction's. A RequestReplyHandler
@@ -72,6 +73,10 @@ type handler struct {
 }
 
 func (h *handler) WithSpec(spec StatefulFunctionSpec) error {
+	if spec.FunctionType == nil {
+		return fmt.Errorf("function type is required")
+	}
+
 	log.Printf("registering Stateful Function %v\n", spec.FunctionType)
 	if _, exists := h.module[spec.FunctionType]; exists {
 		err := fmt.Errorf("failed to register Stateful Function %s, there is already a spec registered under that type", spec.FunctionType)
diff --git a/statefun-sdk-go/v3/pkg/statefun/handler_test.go b/statefun-sdk-go/v3/pkg/statefun/handler_test.go
index e315c4c5d..a6708df0a 100644
--- a/statefun-sdk-go/v3/pkg/statefun/handler_test.go
+++ b/statefun-sdk-go/v3/pkg/statefun/handler_test.go
@@ -4,9 +4,10 @@ import (
 	"context"
 	"testing"
 
-	"github.com/apache/flink-statefun/statefun-sdk-go/v3/pkg/statefun/internal/protocol"
 	"github.com/stretchr/testify/assert"
 	"google.golang.org/protobuf/proto"
+
+	"github.com/apache/flink-statefun/statefun-sdk-go/v3/pkg/statefun/internal/protocol"
 )
 
 // helper to create a protocol Address from an Address
@@ -87,3 +88,20 @@ func TestStatefunHandler_WithCaller_ContextCallerIsCorrect(t *testing.T) {
 	err := invokeStatefulFunction(context.Background(), &target, &caller, nil, StatefulFunctionPointer(statefulFunction))
 	assert.Nil(t, err)
 }
+
+func TestStatefulFunctionsBuilder_FunctionTypeRequired(t *testing.T) {
+	caller := Address{FunctionType: TypeNameFrom("namespace/function2"), Id: "2"}
+
+	statefulFunction := func(ctx Context, message Message) error {
+		assert.Equal(t, caller.String(), ctx.Caller().String())
+		return nil
+	}
+
+	builder := StatefulFunctionsBuilder()
+	err := builder.WithSpec(StatefulFunctionSpec{
+		Function: StatefulFunctionPointer(statefulFunction),
+	})
+
+	assert.Error(t, err)
+	assert.Contains(t, err.Error(), "function type is required")
+}