diff --git a/fastembed/text/custom_text_embedding.py b/fastembed/text/custom_text_embedding.py
index 118b7313..04715860 100644
--- a/fastembed/text/custom_text_embedding.py
+++ b/fastembed/text/custom_text_embedding.py
@@ -129,6 +129,8 @@ def add_model(
         normalization: bool,
         output_name: str | None = None,
     ) -> None:
+        if output_name is not None and (not isinstance(output_name, str) or not output_name):
+            raise ValueError("output_name must be a non-empty string or None")
         cls.SUPPORTED_MODELS.append(model_description)
         cls.POSTPROCESSING_MAPPING[model_description.model] = PostprocessingConfig(
             pooling=pooling, normalization=normalization, output_name=output_name
diff --git a/tests/test_custom_models.py b/tests/test_custom_models.py
index 38751bd6..05b1f7f4 100644
--- a/tests/test_custom_models.py
+++ b/tests/test_custom_models.py
@@ -138,6 +138,58 @@ def test_text_custom_model_output_name():
         delete_model_cache(model.model._model_dir)
 
 
+@pytest.mark.parametrize("output_name", ["", 123, [], False])
+def test_invalid_output_name_preserves_registry_and_allows_retry(output_name):
+    model_name = "custom/output-validation"
+    TextEmbedding.add_custom_model(
+        "custom/existing-output",
+        pooling=PoolingType.DISABLED,
+        normalization=False,
+        sources=ModelSource(hf="local/existing-output"),
+        dim=1,
+    )
+    existing_models = list(CustomTextEmbedding.SUPPORTED_MODELS)
+    existing_mapping = dict(CustomTextEmbedding.POSTPROCESSING_MAPPING)
+
+    with pytest.raises(ValueError, match="output_name"):
+        TextEmbedding.add_custom_model(
+            model_name,
+            pooling=PoolingType.DISABLED,
+            normalization=False,
+            sources=ModelSource(hf="local/output-validation"),
+            dim=1,
+            output_name=output_name,
+        )
+
+    assert CustomTextEmbedding.SUPPORTED_MODELS == existing_models
+    assert CustomTextEmbedding.POSTPROCESSING_MAPPING == existing_mapping
+    TextEmbedding.add_custom_model(
+        model_name,
+        pooling=PoolingType.DISABLED,
+        normalization=False,
+        sources=ModelSource(hf="local/output-validation"),
+        dim=1,
+        output_name="sentence_embedding",
+    )
+    assert CustomTextEmbedding.POSTPROCESSING_MAPPING[model_name].output_name == (
+        "sentence_embedding"
+    )
+
+
+@pytest.mark.parametrize("output_name", [None, "sentence_embedding", " output "])
+def test_valid_output_name_preserves_value(output_name):
+    model_name = "custom/valid-output"
+    TextEmbedding.add_custom_model(
+        model_name,
+        pooling=PoolingType.DISABLED,
+        normalization=False,
+        sources=ModelSource(hf="local/valid-output"),
+        dim=1,
+        output_name=output_name,
+    )
+    assert CustomTextEmbedding.POSTPROCESSING_MAPPING[model_name].output_name == output_name
+
+
 def test_cross_encoder_custom_model():
     is_ci = os.getenv("CI")
     custom_model_name = "Xenova/ms-marco-MiniLM-L-4-v2"