Comfy-Org/ComfyUI · error · Exception

Field '{field_name}' cannot be empty.

Error message

Field '{field_name}' cannot be empty.

What it means

Raised by validate_string() in comfy_api_nodes/util/validation_utils.py when the string input (default field name 'prompt') is None. It is a plain Exception (not ValueError) raised before any API call, so workflow users see it as node execution failure. Nearly every text-input API node (Gemini, OpenAI, Kling, Recraft, LTX, etc.) routes prompts through this helper.

Source

Thrown at comfy_api_nodes/util/validation_utils.py:182

) -> None:
    sr = int(audio["sample_rate"])
    dur = int(audio["waveform"].shape[-1]) / sr
    eps = 1.0 / sr
    if min_duration is not None and dur + eps < min_duration:
        raise ValueError(f"Audio duration must be at least {min_duration}s, got {dur + eps:.2f}s")
    if max_duration is not None and dur - eps > max_duration:
        raise ValueError(f"Audio duration must be at most {max_duration}s, got {dur - eps:.2f}s")


def validate_string(
    string: str,
    strip_whitespace=True,
    field_name="prompt",
    min_length=None,
    max_length=None,
):
    if string is None:
        raise Exception(f"Field '{field_name}' cannot be empty.")
    if strip_whitespace:
        string = string.strip()
    if min_length and len(string) < min_length:
        raise Exception(
            f"Field '{field_name}' cannot be shorter than {min_length} characters; was {len(string)} characters long."
        )
    if max_length and len(string) > max_length:
        raise Exception(
            f" Field '{field_name} cannot be longer than {max_length} characters; was {len(string)} characters long."
        )


def validate_container_format_is_mp4(video: Input.Video) -> None:
    """Validates video container format is MP4."""
    container_format = video.get_container_format()
    if container_format not in ["mp4", "mov,mp4,m4a,3gp,3g2,mj2"]:
        raise ValueError(f"Only MP4 container format supported. Got: {container_format}")

View on GitHub (pinned to 1c6d8d45b3)

Solutions

1. Set an actual string on the node's text input — type the prompt or connect a text-producing node.
2. If building the prompt API-side, guard upstream: ensure the key exists and is a non-None string before queueing.
3. Check the immediately upstream node for None output (e.g. an interrogate/caption node that failed silently) and fix or bypass it.
4. If None is legitimately possible in your workflow, branch on it with a switch node that substitutes a placeholder string.

Example fix

# before
validate_string(None, field_name='prompt')  # Exception: Field 'prompt' cannot be empty.

# after
validate_string(prompt or '', field_name='prompt')
# or simply ensure prompt is a real string before the call

if prompt is None:
    raise ValueError('prompt input is unset')

def is_prompt_text(v) -> bool:
    return isinstance(v, str)

try: validate_string(prompt, field_name='prompt') except Exception as e: raise UserVisibleError(str(e)) from e

When it happens

Trigger: A node's text input is literally None — e.g. an optional upstream conditioning/text node produced None, a Python caller invoked the node function with prompt=None, or a widget value was left unset in a programmatically built workflow JSON.

Common situations: Workflows assembled by scripts where the 'prompt' key is omitted from inputs dict (the node receives None via INPUT_TYPES defaults), or a text node whose upstream supplier returned None (empty caption, failed extraction). Also seen when combo widgets are switched and the stored value becomes invalid.

Related errors

• The prompt references @Audio{max_tag}, but reference mode is
• Field '{field_name}' cannot be shorter than {min_length} cha
• Field '{field_name} cannot be longer than {max_length} char
• Connect at least one keyframe image.
• Spreading {len(images)} images across the clip needs an expl

AI-assisted analysis of Comfy-Org/ComfyUI@1c6d8d45b3 (2026-08-14). Data as JSON: /api/errors/3c639bf4da001e4c. Report an issue: GitHub.