diff --git a/src/roboticstoolbox/blocks/arm.py b/src/roboticstoolbox/blocks/arm.py
index 9fd26ae1f..a3e7c2284 100644
--- a/src/roboticstoolbox/blocks/arm.py
+++ b/src/roboticstoolbox/blocks/arm.py
@@ -1463,13 +1463,13 @@ def __init__(self, robot: Any, q0: Any = None, **blockargs: Any) -> None:
         else:
             q0 = smb.getvector(q0, robot.n)
         self._x0 = np.r_[q0, np.zeros((robot.n,))]
-        self._qdd = None
+        self._qdd = np.zeros((robot.n,))  # placeholder until the first deriv()
 
     def output(self, t: Any, inports: Any, x: Any) -> Any:
         n = self.robot.n
         q = x[:n]
         qd = x[n:]
-        qdd = self._qdd  # from last deriv
+        qdd = self._qdd  # from last deriv(), or the zero placeholder
         return [q, qd, qdd]
 
     def deriv(self, t: Any, inports: Any, x: Any) -> Any:
diff --git a/tests/test_blocks.py b/tests/test_blocks.py
index 46576cf03..30d48e274 100644
--- a/tests/test_blocks.py
+++ b/tests/test_blocks.py
@@ -128,6 +128,16 @@ def test_fdyn(self):
         nt.assert_equal(block.test_output(tau, x=x)[0], q)
         nt.assert_equal(block.getstate0(), x)
 
+    def test_fdyn_qdd_before_deriv(self):
+        # bdsim records the type of each output when it compiles a diagram, so qdd
+        # must be an array of the right shape even before the first deriv() call
+        robot = rtb.models.DH.Puma560()
+        q = robot.configs["qn"]
+        block = FDyn(robot, q)
+
+        qdd = block.test_output([0] * 6, x=np.r_[q, np.zeros((6,))])[2]
+        self.assertEqual(np.shape(qdd), (6,))
+
     @unittest.skip
     def test_fdyn_x(self):
         robot = rtb.models.DH.Puma560()