diff --git a/matlab/vl_compilenn.m b/matlab/vl_compilenn.m
index 03b2e3d1..aeeb9953 100644
--- a/matlab/vl_compilenn.m
+++ b/matlab/vl_compilenn.m
@@ -337,7 +337,7 @@ function vl_compilenn(varargin)
   flags.base{end+1} = '-g' ;
   flags.base{end+1} = '-DDEBUG' ;
 else
-  flags.base{end+1} = '-O' ;
+  flags.base{end+1} = '-O0' ;
   flags.base{end+1} = '-DNDEBUG' ;
 end