sqldata.diff

diff --git a/coverage/sqldata.py b/coverage/sqldata.py
index 5e3d6790..2b5dae1f 100644
--- a/coverage/sqldata.py
+++ b/coverage/sqldata.py
@@ -126,15 +126,17 @@ class NumbitsUnionAgg:
     """SQLite aggregate function for computing union of numbits."""
 
     def __init__(self) -> None:
-        self.result = b""
+        self.result = 0
+        self.len = 0
 
     def step(self, value: bytes) -> None:
         """Process one value in the aggregation."""
-        self.result = numbits_union(self.result, value)
+        self.result |= int.from_bytes(value, "little")
+        self.len = max(self.len, len(value))
 
     def finalize(self) -> bytes:
         """Return the final aggregated result."""
-        return self.result
+        return self.result.to_bytes(self.len, "little")
 
 
 class CoverageData: