diff --git a/src/group_map.rs b/src/group_map.rs
index 8891f95ac..3dcee83af 100644
--- a/src/group_map.rs
+++ b/src/group_map.rs
@@ -22,10 +22,11 @@ where
     lookup
 }
 
-pub fn into_group_map_by<I, K, V>(iter: I, f: impl Fn(&V) -> K) -> HashMap<K, Vec<V>>
+pub fn into_group_map_by<I, K, V, F>(iter: I, mut f: F) -> HashMap<K, Vec<V>>
 where
     I: Iterator<Item = V>,
     K: Hash + Eq,
+    F: FnMut(&V) -> K,
 {
     into_group_map(iter.map(|v| (f(&v), v)))
 }
diff --git a/src/lib.rs b/src/lib.rs
index 64f2b5ac4..7171b1dc0 100644
--- a/src/lib.rs
+++ b/src/lib.rs
@@ -3257,7 +3257,7 @@ pub trait Itertools: Iterator {
     where
         Self: Iterator<Item = V> + Sized,
         K: Hash + Eq,
-        F: Fn(&V) -> K,
+        F: FnMut(&V) -> K,
     {
         group_map::into_group_map_by(self, f)
     }