diff --git a/src/combinator/mod.rs b/src/combinator/mod.rs
index 2ba84d1..7071cc7 100644
--- a/src/combinator/mod.rs
+++ b/src/combinator/mod.rs
@@ -205,11 +205,11 @@ where
 /// ```
 pub fn flat_map<I, O1, O2, E: ParseError<I>, F, G, H>(
   mut parser: F,
-  applied_parser: G,
+  mut applied_parser: G,
 ) -> impl FnMut(I) -> IResult<I, O2, E>
 where
   F: Parser<I, O1, E>,
-  G: Fn(O1) -> H,
+  G: FnMut(O1) -> H,
   H: Parser<I, O2, E>,
 {
   move |input: I| {
diff --git a/src/internal.rs b/src/internal.rs
index 762f3d8..8a3c6a1 100644
--- a/src/internal.rs
+++ b/src/internal.rs
@@ -259,7 +259,7 @@ pub trait Parser<I, O, E> {
   /// Creates a second parser from the output of the first one, then apply over the rest of the input
   fn flat_map<G, H, O2>(self, g: G) -> FlatMap<Self, G, O>
   where
-    G: Fn(O) -> H,
+    G: FnMut(O) -> H,
     H: Parser<I, O2, E>,
     Self: core::marker::Sized,
   {