don't partially regen derivative_cache if derivative is disabled

src/function.rs

@@ -130,6 +130,7 @@ impl FunctionEntry {
             );
 
             if self.derivative_cache.is_some() {
+                if self.derivative {
                     let derivative_cache = self.derivative_cache.as_ref().unwrap();
 
                     self.derivative_cache = Some(
@@ -144,6 +145,9 @@ impl FunctionEntry {
                             })
                             .collect(),
                     );
+                } else {
+                    self.derivative_cache = None;
+                }
             }
         } else {
             self.back_cache = None;