diff --git a/chainladder/utils/sparse.py b/chainladder/utils/sparse.py
index 2de836b8..e3dee306 100644
--- a/chainladder/utils/sparse.py
+++ b/chainladder/utils/sparse.py
@@ -66,7 +66,8 @@ def cumprod(a, axis=None, dtype=None, out=None):
     return array(np.cumprod(a.todense(), axis=axis, dtype=dtype, out=out))
 
 
-def floor(x):
+def floor(x: COO) -> COO:
+    x = x.copy()
     x.data = np.floor(x.data)
     return x
 
diff --git a/chainladder/utils/tests/test_sparse.py b/chainladder/utils/tests/test_sparse.py
index c8a2b211..d92cdbf3 100644
--- a/chainladder/utils/tests/test_sparse.py
+++ b/chainladder/utils/tests/test_sparse.py
@@ -97,9 +97,10 @@ def test_floor_rounds_down() -> None:
     np.testing.assert_array_equal(result.todense(), [1.0, 2.0, -1.0])
 
 
-def test_floor_mutates_in_place() -> None:
+def test_floor_returns_copy() -> None:
     """
-    Checks in-place mutation of floor function.
+    Checks that floor returns a copy and does not mutate its input,
+    mirroring np.floor (see issue #740).
 
     Returns
     -------
@@ -107,4 +108,6 @@ def test_floor_mutates_in_place() -> None:
     """
     a = array([1.2, 2.7, -0.3])
     result: COO = floor(a)
-    assert result is a
+    assert result is not a
+    np.testing.assert_array_equal(result.todense(), [1.0, 2.0, -1.0])
+    np.testing.assert_array_equal(a.todense(), [1.2, 2.7, -0.3])