diff --git a/crates/compiler/builtins/bitcode/src/list.zig b/crates/compiler/builtins/bitcode/src/list.zig
index eef76bfbaf..37d29fbe1d 100644
--- a/crates/compiler/builtins/bitcode/src/list.zig
+++ b/crates/compiler/builtins/bitcode/src/list.zig
@@ -740,9 +740,11 @@ fn swapElements(source_ptr: [*]u8, element_width: usize, index_1: usize, index_2
 }
 
 pub fn listConcat(list_a: RocList, list_b: RocList, alignment: u32, element_width: usize) callconv(.C) RocList {
-    if (list_a.isEmpty()) {
-        return list_b;
-    } else if (list_b.isEmpty()) {
+    // NOTE we always use list_a! because it is owned, we must consume it, and it may have unused capacity
+    if (list_b.isEmpty()) {
+        // we must consume this list. Even though it has no elements, it could still have capacity
+        list_b.deinit(usize);
+
         return list_a;
     } else if (list_a.isUnique()) {
         const total_length: usize = list_a.len() + list_b.len();