diff --git a/mne/source_estimate.py b/mne/source_estimate.py
index b5e5e0b7f73..19b23da7d60 100644
--- a/mne/source_estimate.py
+++ b/mne/source_estimate.py
@@ -1645,7 +1645,7 @@ def save_as_surface(self, fname, src, *, scale=1, scale_rr=1e3):
             data = getattr(self, f"{hemi}_data") * scale
             ts = [
                 nib.gifti.gifti.GiftiDataArray(
-                    data=data[:, idx],
+                    data=data[:, idx].astype(np.float32),
                     intent="NIFTI_INTENT_POINTSET",
                     datatype="NIFTI_TYPE_FLOAT32",
                 )