diff --git a/python/_tskitmodule.c b/python/_tskitmodule.c
index dabf0db2a4..0d89a677dc 100644
--- a/python/_tskitmodule.c
+++ b/python/_tskitmodule.c
@@ -13254,10 +13254,13 @@ LsHmm_forward_matrix(LsHmm *self, PyObject *args)
             PyExc_ValueError, "haplotype array must have dimension (num_sites,)");
         goto out;
     }
+    // clang-format off
     Py_BEGIN_ALLOW_THREADS err
         = tsk_ls_hmm_forward(self->ls_hmm, PyArray_DATA(haplotype_array),
             compressed_matrix->compressed_matrix, TSK_NO_INIT);
-    Py_END_ALLOW_THREADS if (err != 0)
+    Py_END_ALLOW_THREADS
+        // clang-format on
+        if (err != 0)
     {
         handle_library_error(err);
         goto out;