diff --git a/tests/multimodal/test_video.py b/tests/multimodal/test_video.py
index 9c24b5f96932..14f9df05e017 100644
--- a/tests/multimodal/test_video.py
+++ b/tests/multimodal/test_video.py
@@ -1755,3 +1755,16 @@ def test_glm5next_read_frames_dense_walk_matches_stock(tmp_path):
         assert abs(round(float(np.asarray(frame).mean())) - idx) <= 1
     # One initial seek, then pure walking -- no re-seek churn.
     assert cap.seeks == 1
+
+
+def test_glmga_video_backend_rejects_unknown_source_fps():
+    """A container reporting 0 fps (VFR/unknown) must raise a clear
+    ValueError instead of ZeroDivisionError."""
+    target = VideoTargetMetadata(num_frames=-1, fps=2, max_duration=300)
+    # Duration may or may not be reported; either path divides by original_fps.
+    for duration in (5.0, 0.0):
+        source = VideoSourceMetadata(
+            total_frames_num=150, original_fps=0.0, duration=duration
+        )
+    with pytest.raises(ValueError, match="unknown frame rate"):
+        GLMGAVideoBackend.compute_frames_index_to_sample(source, target)
diff --git a/vllm/multimodal/video.py b/vllm/multimodal/video.py
index 2cc2aa98877a..cf1ade3f8392 100644
--- a/vllm/multimodal/video.py
+++ b/vllm/multimodal/video.py
@@ -813,6 +813,14 @@ def compute_frames_index_to_sample(
         max_frame_idx = source.total_frames_num - 1
         max_frames = min(kwargs.get("max_frames", cls._MAX_FRAMES), cls._MAX_FRAMES)
 
+        # vLLM reports original_fps == 0 for clips with unknown/variable fps
+        # (VFR, malformed, streaming); fail loudly instead of dividing by zero.
+        if original_fps <= 0:
+            raise ValueError(
+                "GLMGA video sampling needs a known source fps, but the "
+                "container reported 0 (variable or unknown frame rate)."
+            )
+
         duration = duration or round(max_frame_idx / original_fps) + 1
 
         extract_t = int(duration * target_fps)