diff --git a/inference/core/workflows/core_steps/formatters/vlm_as_detector/v1.py b/inference/core/workflows/core_steps/formatters/vlm_as_detector/v1.py
index e47409e7c9..bfdf84dc3f 100644
--- a/inference/core/workflows/core_steps/formatters/vlm_as_detector/v1.py
+++ b/inference/core/workflows/core_steps/formatters/vlm_as_detector/v1.py
@@ -224,6 +224,15 @@ def validate(self) -> "BlockManifest":
             raise ValueError(
                 "Must pass list of classes to this block when using gemini or claude"
             )
+        if (
+            self.model_type == "florence-2"
+            and self.task_type == "open-vocabulary-object-detection"
+            and self.classes is None
+        ):
+            raise ValueError(
+                "Must pass list of classes to this block when using florence-2 "
+                "with open-vocabulary-object-detection task"
+            )
 
         return self
 
diff --git a/inference/core/workflows/core_steps/formatters/vlm_as_detector/v2.py b/inference/core/workflows/core_steps/formatters/vlm_as_detector/v2.py
index 73f9bde112..918226ec19 100644
--- a/inference/core/workflows/core_steps/formatters/vlm_as_detector/v2.py
+++ b/inference/core/workflows/core_steps/formatters/vlm_as_detector/v2.py
@@ -244,6 +244,15 @@ def validate(self) -> "BlockManifest":
             raise ValueError(
                 "Must pass list of classes to this block when using gemini or claude"
             )
+        if (
+            self.model_type == "florence-2"
+            and self.task_type == "open-vocabulary-object-detection"
+            and self.classes is None
+        ):
+            raise ValueError(
+                "Must pass list of classes to this block when using florence-2 "
+                "with open-vocabulary-object-detection task"
+            )
 
         return self