diff --git a/tinycloud-node-server/src/storage/size.rs b/tinycloud-node-server/src/storage/size.rs
index 9fac85d6..19700cc8 100644
--- a/tinycloud-node-server/src/storage/size.rs
+++ b/tinycloud-node-server/src/storage/size.rs
@@ -1,8 +1,4 @@
-use std::{
-    collections::HashMap,
-    ops::{AddAssign, SubAssign},
-    sync::Arc,
-};
+use std::{collections::HashMap, ops::AddAssign, sync::Arc};
 use tinycloud_auth::resource::SpaceId;
 use tokio::sync::RwLock;
 
@@ -23,7 +19,7 @@ impl SpaceSizes {
     }
     pub async fn decrement_size(&self, space: &SpaceId, size: u64) {
         if let Some(s) = self.0.write().await.get_mut(space) {
-            s.sub_assign(size)
+            *s = s.saturating_sub(size);
         }
     }
     pub async fn get_size(&self, space: &SpaceId) -> Option<u64> {
@@ -36,3 +32,23 @@ impl From<HashMap<SpaceId, u64>> for SpaceSizes {
         Self(Arc::new(RwLock::new(map)))
     }
 }
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    #[tokio::test]
+    async fn decrement_floors_at_zero() {
+        let space: SpaceId =
+            "tinycloud:pkh:eip155:1:0x1111111111111111111111111111111111111111:default"
+                .parse()
+                .unwrap();
+        let sizes = SpaceSizes::new();
+        sizes.init_size(space.clone()).await;
+        sizes.increment_size(&space, 10).await;
+        sizes.decrement_size(&space, 25).await;
+        assert_eq!(sizes.get_size(&space).await, Some(0));
+        sizes.decrement_size(&space, 1).await;
+        assert_eq!(sizes.get_size(&space).await, Some(0));
+    }
+}