diff --git a/newsfragments/701.misc b/newsfragments/701.misc
new file mode 100644
index 000000000..253df9d66
--- /dev/null
+++ b/newsfragments/701.misc
@@ -0,0 +1 @@
+Relax conditions when creating time of flight interpolators to allow time of flight frames to start from zero.
diff --git a/src/dxtbx/model/tof_helpers.py b/src/dxtbx/model/tof_helpers.py
index 5417f6e82..a8f5985f7 100644
--- a/src/dxtbx/model/tof_helpers.py
+++ b/src/dxtbx/model/tof_helpers.py
@@ -26,8 +26,8 @@ def frame_to_tof_interpolator(frames: List[float], tof: List[float]) -> interp1d
     ToF can vary nonlinearly with frame number.
     A cubic spline is used to better account for these cases.
     """
-    assert min(frames) > 0
-    assert min(tof) > 0
+    assert min(frames) >= 0
+    assert min(tof) >= 0
     assert len(frames) == len(tof)
     assert all(i < j for i, j in zip(frames, frames[1:]))
     assert all(i < j for i, j in zip(tof, tof[1:]))
@@ -40,8 +40,8 @@ def tof_to_frame_interpolator(tof: List[float], frames: List[float]) -> interp1d
     ToF can vary nonlinearly with frame number.
     A cubic spline is used to better account for these cases.
     """
-    assert min(frames) > 0
-    assert min(tof) > 0
+    assert min(frames) >= 0
+    assert min(tof) >= 0
     assert len(frames) == len(tof)
     assert all(i < j for i, j in zip(frames, frames[1:]))
     assert all(i < j for i, j in zip(tof, tof[1:]))