diff --git a/crates/polars-core/src/frame/group_by/proxy.rs b/crates/polars-core/src/frame/group_by/proxy.rs
index 70a478f2da53..d8bc3b4c60e3 100644
--- a/crates/polars-core/src/frame/group_by/proxy.rs
+++ b/crates/polars-core/src/frame/group_by/proxy.rs
@@ -367,19 +367,16 @@ impl GroupsProxy {
         }
     }
 
-    pub fn take_group_lasts(self) -> Vec<IdxSize> {
+    /// # Safety
+    /// This will not do any bounds checks. The caller must ensure
+    /// all groups have members.
+    pub unsafe fn take_group_lasts(self) -> Vec<IdxSize> {
         match self {
-            GroupsProxy::Idx(groups) => {
-                groups
-                    .all
-                    .iter()
-                    .map(|idx| {
-                        // safety:
-                        // idx has at least one eletment, so -1 is always in bounds
-                        unsafe { *idx.get_unchecked(idx.len() - 1) }
-                    })
-                    .collect()
-            },
+            GroupsProxy::Idx(groups) => groups
+                .all
+                .iter()
+                .map(|idx| *idx.get_unchecked(idx.len() - 1))
+                .collect(),
             GroupsProxy::Slice { groups, .. } => groups
                 .into_iter()
                 .map(|[first, len]| first + len - 1)
diff --git a/crates/polars-ops/src/series/ops/is_last_distinct.rs b/crates/polars-ops/src/series/ops/is_last_distinct.rs
index 9cb00799dacf..57c388f2c5fc 100644
--- a/crates/polars-ops/src/series/ops/is_last_distinct.rs
+++ b/crates/polars-ops/src/series/ops/is_last_distinct.rs
@@ -149,7 +149,8 @@ where
 #[cfg(feature = "dtype-struct")]
 fn is_last_distinct_struct(s: &Series) -> PolarsResult<BooleanChunked> {
     let groups = s.group_tuples(true, false)?;
-    let last = groups.take_group_lasts();
+    // SAFETY: all groups have at least a single member
+    let last = unsafe { groups.take_group_lasts() };
     let mut out = MutableBitmap::with_capacity(s.len());
     out.extend_constant(s.len(), false);
 
@@ -165,7 +166,8 @@ fn is_last_distinct_struct(s: &Series) -> PolarsResult<BooleanChunked> {
 #[cfg(feature = "group_by_list")]
 fn is_last_distinct_list(ca: &ListChunked) -> PolarsResult<BooleanChunked> {
     let groups = ca.group_tuples(true, false)?;
-    let last = groups.take_group_lasts();
+    // SAFETY: all groups have at least a single member
+    let last = unsafe { groups.take_group_lasts() };
     let mut out = MutableBitmap::with_capacity(ca.len());
     out.extend_constant(ca.len(), false);