diff --git a/lslopt/lsloptimizer.py b/lslopt/lsloptimizer.py
index cb48459..a76569b 100644
--- a/lslopt/lsloptimizer.py
+++ b/lslopt/lsloptimizer.py
@@ -196,6 +196,9 @@ class optimizer(object):
                 else:
                     # Call it
                     val = self.functions[code[2]][2](*tuple(x[2] for x in code[3]))
+                    if not self.foldtabs and isinstance(val, unicode) and '\t' in val:
+                        warning('WARNING: Tab in function result and foldtabs option not used.')
+                        return
                     code[:] = [CONSTANT, code[1], val]
             return
 
@@ -360,6 +363,8 @@ class optimizer(object):
         if 'optimize' not in options:
             return
 
+        self.foldtabs = 'foldtabs' in options
+
         # TODO: Add option to handle local jumps properly.
 
         self.functions = functions
diff --git a/main.py b/main.py
index 8494c9e..92678d5 100644
--- a/main.py
+++ b/main.py
@@ -35,7 +35,7 @@ Options (* means not implemented):
                        directives like: # 123 "filename".
   optimize             Runs the optimizer.
   optsigns             Optimize signs and float as int.
-  * foldtabs           Tabs can't be copy-pasted, so they aren't optimized by
+  foldtabs             Tabs can't be copy-pasted, so they aren't optimized by
                        default. But with support from the viewer, they can be
                        folded too and make it to the uploaded source. This
                        option overrides that check, enabling optimization of