diff --git a/dpti/lib/utils.py b/dpti/lib/utils.py
index 354e426..914e8b4 100644
--- a/dpti/lib/utils.py
+++ b/dpti/lib/utils.py
@@ -206,14 +206,20 @@ def copy_file_list(file_list, from_path, to_path):
 
 
 def block_avg(inp, skip=0, block_size=10):
+    """Return a block average and error from complete post-skip blocks."""
+    if block_size <= 0:
+        raise ValueError("block_size must be a positive integer")
     inp = inp[skip:]
+    if len(inp) < block_size:
+        raise ValueError(
+            "block_avg requires at least one complete block after skip: "
+            f"got {len(inp)} samples for block_size={block_size}"
+        )
+    nblocks = len(inp) // block_size
     data_chunks = [
-        list(inp[i : i + block_size]) for i in range(0, len(inp), block_size)
+        list(inp[i : i + block_size])
+        for i in range(0, nblocks * block_size, block_size)
     ]
-    nblocks = len(data_chunks)
-    if len(data_chunks[-1]) != block_size:
-        nblocks -= 1
-        data_chunks = data_chunks[:nblocks]
     assert len(data_chunks) == nblocks
     # naive avg
     naive_avg = np.average(inp)
diff --git a/tests/test_lib_utils.py b/tests/test_lib_utils.py
index f801ff6..502eb4c 100644
--- a/tests/test_lib_utils.py
+++ b/tests/test_lib_utils.py
@@ -116,6 +116,14 @@ def test_normal(self):
         self.assertAlmostEqual(avg1, avg2, places=8)
         self.assertAlmostEqual(err1, err2, places=8)
 
+    def test_rejects_partial_only_input(self):
+        with self.assertRaisesRegex(ValueError, "at least one complete block"):
+            block_avg(np.arange(5), block_size=10)
+
+    def test_rejects_empty_post_skip_input(self):
+        with self.assertRaisesRegex(ValueError, "got 0 samples"):
+            block_avg(np.arange(5), skip=5, block_size=2)
+
 
 class TestIntegrateRangeHti(unittest.TestCase):
     def setUp(self):