diff --git a/crates/voicevox_core/src/infer.rs b/crates/voicevox_core/src/infer.rs
index d5a55cfea..1fccadf0a 100644
--- a/crates/voicevox_core/src/infer.rs
+++ b/crates/voicevox_core/src/infer.rs
@@ -2,7 +2,7 @@ mod model_file;
 pub(crate) mod runtimes;
 pub(crate) mod signatures;
 
-use std::{fmt::Debug, marker::PhantomData, sync::Arc};
+use std::{collections::HashMap, fmt::Debug, marker::PhantomData, sync::Arc};
 
 use derive_new::new;
 use enum_map::{Enum, EnumMap};
@@ -29,13 +29,13 @@ pub(crate) trait RunBuilder<'a>: From<&'a mut Self::Session> {
     fn input(&mut self, tensor: Array<impl InputScalar, impl Dimension + 'static>) -> &mut Self;
 }
 
-pub(crate) trait InputScalar: LinalgScalar + Debug + sealed::OnnxruntimeInputScalar {}
+pub(crate) trait InputScalar: LinalgScalar + Debug + sealed::InputScalar {}
 
 impl InputScalar for i64 {}
 impl InputScalar for f32 {}
 
 pub(crate) trait Signature: Sized + Send + 'static {
-    type Kind: Enum;
+    type Kind: Enum + Copy;
     type Output;
     const KIND: Self::Kind;
     fn input<'a, 'b>(self, ctx: &'a mut impl RunBuilder<'b>);
@@ -49,7 +49,7 @@ pub(crate) struct SessionSet<K: Enum, R: InferenceRuntime>(
     EnumMap<K, Arc<std::sync::Mutex<R::Session>>>,
 );
 
-impl<K: Enum, R: InferenceRuntime> SessionSet<K, R> {
+impl<K: Enum + Copy, R: InferenceRuntime> SessionSet<K, R> {
     pub(crate) fn new(
         model_bytes: &EnumMap<K, Vec<u8>>,
         mut options: impl FnMut(K) -> SessionOptions,
@@ -58,12 +58,12 @@ impl<K: Enum, R: InferenceRuntime> SessionSet<K, R> {
             .iter()
             .map(|(k, m)| {
                 let sess = R::Session::new(|| model_file::decrypt(m), options(k))?;
-                Ok(Some(Arc::new(std::sync::Mutex::new(sess))))
+                Ok((k.into_usize(), std::sync::Mutex::new(sess).into()))
             })
-            .collect::<anyhow::Result<Vec<_>>>()?;
+            .collect::<anyhow::Result<HashMap<_, _>>>()?;
 
         Ok(Self(EnumMap::<K, _>::from_fn(|k| {
-            sessions[k.into_usize()].take().expect("should exist")
+            sessions.remove(&k.into_usize()).expect("should exist")
         })))
     }
 }
@@ -105,11 +105,15 @@ pub(crate) struct SessionOptions {
 pub(crate) struct DecryptModelError;
 
 mod sealed {
+    pub(crate) trait InputScalar: OnnxruntimeInputScalar {}
+
+    impl InputScalar for i64 {}
+    impl InputScalar for f32 {}
+
     pub(crate) trait OnnxruntimeInputScalar:
         onnxruntime::TypeToTensorElementDataType
     {
     }
 
-    impl OnnxruntimeInputScalar for i64 {}
-    impl OnnxruntimeInputScalar for f32 {}
+    impl<T: onnxruntime::TypeToTensorElementDataType> OnnxruntimeInputScalar for T {}
 }