diff --git a/nistats/hemodynamic_models.py b/nistats/hemodynamic_models.py
index 85f75c63..e71a7042 100644
--- a/nistats/hemodynamic_models.py
+++ b/nistats/hemodynamic_models.py
@@ -281,7 +281,9 @@ def _sample_condition(exp_condition, frame_times, oversampling=50,
     tmax = len(hr_frame_times)
     regressor = np.zeros_like(hr_frame_times).astype(np.float)
     t_onset = np.minimum(np.searchsorted(hr_frame_times, onsets), tmax - 1)
-    regressor[t_onset] += values
+    for t, v in zip(t_onset, values):
+        regressor[t] += v
+
     t_offset = np.minimum(np.searchsorted(hr_frame_times, onsets + durations),
                           tmax - 1)
 
@@ -290,7 +292,9 @@ def _sample_condition(exp_condition, frame_times, oversampling=50,
         if t < (tmax - 1) and t == t_onset[i]:
             t_offset[i] += 1
 
-    regressor[t_offset] -= values
+    for t, v in zip(t_offset, values):
+        regressor[t] -= v
+
     regressor = np.cumsum(regressor)
 
     return regressor, hr_frame_times