diff --git a/src/array/dictionary/mod.rs b/src/array/dictionary/mod.rs
index 53243208fb..8dc1bbd7c9 100644
--- a/src/array/dictionary/mod.rs
+++ b/src/array/dictionary/mod.rs
@@ -24,7 +24,11 @@ use super::{new_empty_array, primitive::PrimitiveArray, Array};
 use super::{new_null_array, specification::check_indexes};
 
 /// Trait denoting [`NativeType`]s that can be used as keys of a dictionary.
-pub trait DictionaryKey: NativeType + TryInto<usize> + TryFrom<usize> {
+/// # Safety
+///
+/// Any implementation of this trait must ensure that `always_fits_usize` only
+/// returns `true` if all values succeeds on `value::try_into::<usize>().unwrap()`.
+pub unsafe trait DictionaryKey: NativeType + TryInto<usize> + TryFrom<usize> {
     /// The corresponding [`IntegerType`] of this key
     const KEY_TYPE: IntegerType;
 
@@ -39,45 +43,46 @@ pub trait DictionaryKey: NativeType + TryInto<usize> + TryFrom<usize> {
         }
     }
 
+    /// If the key type always can be converted to `usize`.
     fn always_fits_usize() -> bool {
         false
     }
 }
 
-impl DictionaryKey for i8 {
+unsafe impl DictionaryKey for i8 {
     const KEY_TYPE: IntegerType = IntegerType::Int8;
 }
-impl DictionaryKey for i16 {
+unsafe impl DictionaryKey for i16 {
     const KEY_TYPE: IntegerType = IntegerType::Int16;
 }
-impl DictionaryKey for i32 {
+unsafe impl DictionaryKey for i32 {
     const KEY_TYPE: IntegerType = IntegerType::Int32;
 }
-impl DictionaryKey for i64 {
+unsafe impl DictionaryKey for i64 {
     const KEY_TYPE: IntegerType = IntegerType::Int64;
 }
-impl DictionaryKey for u8 {
+unsafe impl DictionaryKey for u8 {
     const KEY_TYPE: IntegerType = IntegerType::UInt8;
 
     fn always_fits_usize() -> bool {
         true
     }
 }
-impl DictionaryKey for u16 {
+unsafe impl DictionaryKey for u16 {
     const KEY_TYPE: IntegerType = IntegerType::UInt16;
 
     fn always_fits_usize() -> bool {
         true
     }
 }
-impl DictionaryKey for u32 {
+unsafe impl DictionaryKey for u32 {
     const KEY_TYPE: IntegerType = IntegerType::UInt32;
 
     fn always_fits_usize() -> bool {
         true
     }
 }
-impl DictionaryKey for u64 {
+unsafe impl DictionaryKey for u64 {
     const KEY_TYPE: IntegerType = IntegerType::UInt64;
 
     #[cfg(target_pointer_width = "64")]
@@ -145,7 +150,7 @@ impl<K: DictionaryKey> DictionaryArray<K> {
             if K::always_fits_usize() {
                 // safety: we just checked that conversion to `usize` always
                 // succeeds
-                unsafe { check_indexes_unchecked(keys.values(), len) }?;
+                unsafe { check_indexes_unchecked(keys.values(), values.len()) }?;
             } else {
                 check_indexes(keys.values(), values.len())?;
             }