diff --git a/ax/preview/api/client.py b/ax/preview/api/client.py
index f489a3664dc..16fed44b1ff 100644
--- a/ax/preview/api/client.py
+++ b/ax/preview/api/client.py
@@ -441,7 +441,7 @@ def attach_data(
         # If no progression is provided assume the data is not timeseries-like and
         # set step=NaN
         data_with_progression = [
-            ({"step": progression if progression is not None else np.nan}, raw_data)
+            ({"step": progression if progression is not None else -1.0}, raw_data)
         ]
 
         trial = assert_is_instance(self._experiment.trials[trial_index], Trial)
diff --git a/ax/preview/api/tests/test_client.py b/ax/preview/api/tests/test_client.py
index 0561bd8f5aa..13e416a882f 100644
--- a/ax/preview/api/tests/test_client.py
+++ b/ax/preview/api/tests/test_client.py
@@ -415,7 +415,7 @@ def test_attach_data(self) -> None:
                         "mean": {0: 1.0},
                         "sem": {0: np.nan},
                         "trial_index": {0: 0},
-                        "step": {0: np.nan},
+                        "step": {0: -1.0},
                     }
                 )
             ),
@@ -440,7 +440,7 @@ def test_attach_data(self) -> None:
                         "mean": {0: 1.0, 1: 2.0},
                         "sem": {0: np.nan, 1: np.nan},
                         "trial_index": {0: 0, 1: 0},
-                        "step": {0: np.nan, 1: 10.0},
+                        "step": {0: -1, 1: 10.0},
                     }
                 )
             ),
@@ -467,7 +467,7 @@ def test_attach_data(self) -> None:
                         "mean": {0: 2.0, 1: 1.0, 2: 2.0},
                         "sem": {0: np.nan, 1: np.nan, 2: np.nan},
                         "trial_index": {0: 0, 1: 0, 2: 0},
-                        "step": {0: 10.0, 1: np.nan, 2: np.nan},
+                        "step": {0: 10.0, 1: -1.0, 2: -1.0},
                     }
                 )
             ),
@@ -513,7 +513,7 @@ def test_complete_trial(self) -> None:
                         "mean": {0: 1.0, 1: 2.0},
                         "sem": {0: np.nan, 1: np.nan},
                         "trial_index": {0: 0, 1: 0},
-                        "step": {0: np.nan, 1: np.nan},
+                        "step": {0: -1.0, 1: -1.0},
                     }
                 )
             ),
@@ -568,7 +568,7 @@ def test_complete_trial(self) -> None:
                         "mean": {0: 1.0},
                         "sem": {0: np.nan},
                         "trial_index": {0: 2},
-                        "step": {0: np.nan},
+                        "step": {0: -1.0},
                     }
                 )
             ),