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)));
 }