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diff --git a/modules/cache/cache_util.c b/modules/cache/cache_util.c
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index 7b7fb45..fbebb1e 100644
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--- a/modules/cache/cache_util.c
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+++ b/modules/cache/cache_util.c
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@@ -1251,8 +1251,10 @@ CACHE_DECLARE(apr_table_t *)ap_cache_cacheable_headers_out(request_rec *r)
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     if (!apr_table_get(headers_out, "Content-Type")
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         && r->content_type) {
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-        apr_table_setn(headers_out, "Content-Type",
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-                       ap_make_content_type(r, r->content_type));
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+        const char *ctype = ap_make_content_type(r, r->content_type);
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+        if (ctype) {
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+            apr_table_setn(headers_out, "Content-Type", ctype);
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+        }
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     }
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     if (!apr_table_get(headers_out, "Content-Encoding")