diff --git a/examples/extern_C/mathext.c b/examples/extern_C/mathext.c
index 4e05bdf..91495b1 100644
--- a/examples/extern_C/mathext.c
+++ b/examples/extern_C/mathext.c
@@ -16,6 +16,6 @@ void st_cos_reset(st_cos_mem *self)
 void st_cos_step(float a, st_cos_out *out, st_cos_mem *self)
 {
   out->o = self->mem[self->i];
-  self->i = (self->i++) % 100;
+  self->i = (self->i+1) % 100;
   self->mem[self->i] = cos(a);
 }