diff --git a/src/solver/modwrap.c b/src/solver/modwrap.c

index a180dba649632c3d9c8d7e812a4bac3a60177d2d..124d4bc4b7b3b313f34deb223afc6267451bf974 100644

--- a/src/solver/modwrap.c

+++ b/src/solver/modwrap.c

@@ -18,6 +18,6 @@ char *solve_equation(const char *eq)

         solve_equation_closure = caml_named_value("solve_equation");

     }

-    value v = copy_string(eq);

-    return strdup(String_val(callback(*solve_equation_closure, v)));

+    value v = caml_copy_string(eq);

+    return strdup(String_val(caml_callback(*solve_equation_closure, v)));

 }