diff --git a/rolldice/rolldice.py b/rolldice/rolldice.py
index 95f7a7f..c18f932 100644
--- a/rolldice/rolldice.py
+++ b/rolldice/rolldice.py
@@ -812,7 +812,7 @@ def roll_dice(roll, *, functions=True, floats=True):
                 num_to_drop = int(drop[3] if drop[3] != '' else 1)
                 assert 1 <= num_to_drop < len(group_result)
 
-                results.append(sum(group_result[:num_to_drop]))
+                results.append(sum(group_result[num_to_drop:]))
                 roll = ','.join([str(i) for i in group_result[num_to_drop:]]) + ' ~~ '  # Same as above.
                 roll += ','.join([str(i) for i in group_result[:num_to_drop]])
                 string.append('[%s]' % roll)
diff --git a/tests/test_drop.py b/tests/test_drop.py
new file mode 100644
index 0000000..05e1939
--- /dev/null
+++ b/tests/test_drop.py
@@ -0,0 +1,36 @@
+from rolldice import roll_dice
+from hypothesis import given, note
+from hypothesis import strategies as st
+
+
+# This is a basic sanity check that avoids the need to parse the explanation string.
+# If you roll N dice, and drop N-1, then the result has to be in the range of the die size.
+@given(
+    roll_count=st.integers(min_value=2, max_value=20),
+    die_size=st.integers(min_value=2, max_value=100),
+    op=st.sampled_from("Xx"),
+)
+def test_result_in_bounds(roll_count, die_size, op):
+    drop_count = roll_count - 1
+    expr = f"{roll_count}d{die_size}{op}{drop_count}"
+    result, explanation = roll_dice(expr)
+    note(f'expr={expr}, explanation={explanation}')
+    assert 1 <= result <= die_size
+
+@given(
+    roll_count=st.integers(min_value=2, max_value=20),
+    die_size=st.integers(min_value=2, max_value=100),
+    op=st.sampled_from("Xx"),
+    data=st.data()
+)
+def test_sum(roll_count, die_size, op, data):
+    drop_count = data.draw(st.integers(min_value=1, max_value=roll_count - 1))
+    expr = f"{roll_count}d{die_size}{op}{drop_count}"
+    result, explanation = roll_dice(expr)
+    included, dropped = explanation.strip('[]').split(' ~~ ')
+    included_rolls = [int(x) for x in included.split(',')]
+    dropped_rolls = [int(x) for x in dropped.split(',')]
+    assert(len(dropped_rolls) == drop_count)
+    assert(len(included_rolls) + len(dropped_rolls) == roll_count)
+    included_sum = sum(included_rolls)
+    assert(result == included_sum)