diff --git a/utils/image3i.m b/utils/image3i.m
index 3065a4c5..01676915 100644
--- a/utils/image3i.m
+++ b/utils/image3i.m
@@ -135,7 +135,7 @@
 if nargin < 3 || handle == 0
     % Make a new surface
     h = surface('XData', x, 'YData', y, 'ZData', z, ...
-                'CData', im, ...
+                'CData', double(im), ...
                 'FaceColor', 'texturemap', ...
                 'EdgeColor', 'none', ...
                 'LineStyle', 'none', ...
@@ -145,7 +145,7 @@
                 'CDataMapping', 'direct');
 else
     % Reuse old surface
-    set(handle, 'XData', x, 'YData', y, 'ZData', z, 'CData', im);
+    set(handle, 'XData', x, 'YData', y, 'ZData', z, 'CData', double(im));
     h = handle;
 end