diff --git a/src/string_mutable.zig b/src/string_mutable.zig
index 02ac07cbb3..c894eaf2e9 100644
--- a/src/string_mutable.zig
+++ b/src/string_mutable.zig
@@ -45,6 +45,7 @@ pub const MutableString = struct {
     }
 
     pub fn write(self: *MutableString, bytes: anytype) !usize {
+        bun.debugAssert(bytes.len == 0 or !bun.isSliceInBuffer(bytes, self.list.allocatedSlice()));
         try self.list.appendSlice(self.allocator, bytes);
         return bytes.len;
     }