diff --git a/.github/workflows/ci.yml b/.github/workflows/ci.yml
index 0b0b09b2..76a87acc 100644
--- a/.github/workflows/ci.yml
+++ b/.github/workflows/ci.yml
@@ -31,7 +31,9 @@ jobs:
         tox_env:
           - docs
           - twine_check
-    runs-on: ubuntu-latest
+    # Use the same Ubuntu release as build.os in .readthedocs.yml, so the
+    # documentation is checked against the libisal that Read the Docs uses.
+    runs-on: ubuntu-22.04
     steps:
       - uses: actions/checkout@v4
         with:
diff --git a/src/isal/isal_zlibmodule.c b/src/isal/isal_zlibmodule.c
index 50e6f740..c62f7280 100644
--- a/src/isal/isal_zlibmodule.c
+++ b/src/isal/isal_zlibmodule.c
@@ -763,7 +763,8 @@ isal_zlib_compressobj_impl(PyObject *module, int level, int method, int wbits,
                                     zdict->buf, (uint32_t)zdict->len);
         if (err == COMP_OK) {
             if (flag == IGZIP_ZLIB && zdict->len != 0) {
-                isal_zlib_header_init(&self->zlib_header);
+                // Set every field instead of calling isal_zlib_header_init,
+                // which ISA-L 2.30 does not have.
                 self->zlib_header.info = hist_bits - 8;
                 self->zlib_header.level = level == 0 ? 0 : 1;
                 self->zlib_header.dict_flag = 1;