diff --git a/tests/multimodal/test_video.py b/tests/multimodal/test_video.py
index 9c24b5f96932..d1d36dc53fa9 100644
--- a/tests/multimodal/test_video.py
+++ b/tests/multimodal/test_video.py
@@ -24,6 +24,7 @@
     GLM46VVideoBackend,
     GLMGAVideoBackend,
     Molmo2VideoBackend,
+    OpenCVDynamicOpenPanguVideoBackend,
     Qwen2VLVideoBackend,
     Qwen3VLVideoBackend,
     VideoBackend,
@@ -1437,6 +1438,27 @@ def test_glm46v_dynamic_fps_thresholds(
     assert indices == sorted(set(indices)), "Indices must be sorted and deduplicated"
 
 
+@pytest.mark.parametrize("fps", [2, -1])
+def test_openpangu_num_frames_sentinel(fps):
+    """`num_frames=-1` (the default / "no cap" sentinel) must sample frames
+    instead of raising `ValueError` from `np.linspace(..., -1)`."""
+    source = VideoSourceMetadata(
+        total_frames_num=100, original_fps=30, duration=100 / 30
+    )
+    target = VideoTargetMetadata(num_frames=-1, fps=fps, max_duration=300)
+
+    indices = OpenCVDynamicOpenPanguVideoBackend.compute_frames_index_to_sample(
+        source, target
+    )
+
+    assert len(indices) > 0
+    assert indices == sorted(indices)
+    assert all(0 <= idx < source.total_frames_num for idx in indices)
+    # fps=-1 means "no fps limit" -> sample every frame.
+    if fps == -1:
+        assert len(indices) == source.total_frames_num
+
+
 def test_glm46v_even_frame_count_enforcement():
     """Test that GLM-4.6V always returns an even number of frames."""
     target = VideoTargetMetadata(num_frames=-1, fps=-1, max_duration=-1)
diff --git a/vllm/multimodal/video.py b/vllm/multimodal/video.py
index 854b5572b3ef..59faf9998acd 100644
--- a/vllm/multimodal/video.py
+++ b/vllm/multimodal/video.py
@@ -1220,14 +1220,20 @@ def compute_frames_index_to_sample(
         # `fps` is the FPS parameter passed in for sampling,
         # -1 indicates that sampling can be performed directly without FPS limitation.
         if fps > 0:
-            # Num_frames is the maximum number of frames to sample.
+            # Num_frames is the maximum number of frames to sample; `num_frames < 0`
+            # means "no cap" and defers entirely to the fps-derived count.
             # If fewer frames are sampled at this sample_fps, the update duration will be longer. # noqa: E501
-            if num_frames >= int(total_duration * fps) + 1:
-                num_frames = int(total_duration * fps) + 1
+            max_num_frames = int(total_duration * fps) + 1
+            if num_frames < 0 or num_frames >= max_num_frames:
+                num_frames = max_num_frames
                 # Under the new maximum frame rate, the video duration of the rightmost frame, # noqa: E501
                 # cannot be calculated for frame 0.
                 total_duration = min(total_duration, (num_frames - 1) / fps)
-        elif fps != -1:
+        elif fps == -1:
+            # No fps limit: `num_frames < 0` means sample every frame.
+            if num_frames < 0:
+                num_frames = total_frames_num
+        else:
             raise ValueError(
                 f"requires dataset fps is -1 or greater than 0 but got {fps}"
             )