diff --git a/CHANGES.md b/CHANGES.md
index dbb90970a..8783208e1 100644
--- a/CHANGES.md
+++ b/CHANGES.md
@@ -1,3 +1,7 @@
+## Unreleased
+
+- http: Fix assertion in `Source.to_string_trim` when `pos <> 0` (mefyl #1017)
+
 ## v6.0.0~beta2 (2024-01-05)
 
 - cohttp-eio: Don't blow up Server on client disconnections. (mefyl #1011)
diff --git a/http/src/http.ml b/http/src/http.ml
index f2139d4c0..dd963a720 100644
--- a/http/src/http.ml
+++ b/http/src/http.ml
@@ -944,7 +944,7 @@ module Parser = struct
              "Http_parser.Source.substring: Index out of bounds., Requested \
               off: %d, len: %d"
              pos len);
-      let last = ref (t.pos + len - 1) in
+      let last = ref (t.pos + pos + len - 1) in
       let pos = ref (t.pos + pos) in
       while is_space (String.unsafe_get t.buffer !pos) do
         incr pos