diff --git a/pytket/tests/circuit_test.py b/pytket/tests/circuit_test.py
index 9a8b6d6a47..6954a6356a 100644
--- a/pytket/tests/circuit_test.py
+++ b/pytket/tests/circuit_test.py
@@ -400,6 +400,29 @@ def test_boxes() -> None:
     assert all(isinstance(box, Op) for box in boxes)
 
 
+def test_u1q_stability() -> None:
+    # https://github.com/CQCL/tket/issues/222
+    u = np.array(
+        [
+            [
+                -1.0000000000000000e00 + 0.0000000000000000e00j,
+                -4.7624091282918654e-10 + 2.0295010872500105e-16j,
+            ],
+            [
+                4.5447577055178555e-10 - 1.4232772405184710e-10j,
+                -9.5429791447115209e-01 + 2.9885697320961047e-01j,
+            ],
+        ]
+    )
+    ubox = Unitary1qBox(u)
+    op = ubox.get_circuit().get_commands()[0].op
+    assert op.type == OpType.TK1
+    a, b, c = op.params
+    assert np.isfinite(a)
+    assert np.isfinite(b)
+    assert np.isfinite(c)
+
+
 def test_custom_gates() -> None:
     a = Symbol("a")
     b = Symbol("b")
diff --git a/tket/src/Gate/Rotation.cpp b/tket/src/Gate/Rotation.cpp
index 7f07caebcc..c4c583766a 100644
--- a/tket/src/Gate/Rotation.cpp
+++ b/tket/src/Gate/Rotation.cpp
@@ -442,6 +442,9 @@ std::vector<double> tk1_angles_from_unitary(const Eigen::Matrix2cd &U) {
   } else {  // general case
     // s0^2 + z0^2 - x0^2 - y0^2 = cos(pi b)
     double t = s0 * s0 + z0 * z0 - x0 * x0 - y0 * y0;
+    // Rounding errors may mean t is outside the domain of acos. Fix this.
+    if (t > +1.) t = +1.;
+    if (t < -1.) t = -1.;
     b = std::acos(t) / PI;
 
     // w.l.o.g. b is in the range (-1,+1).