diff --git a/src/endpoints/sources.js b/src/endpoints/sources.js
index e963f5167..a703948ed 100644
--- a/src/endpoints/sources.js
+++ b/src/endpoints/sources.js
@@ -305,40 +305,53 @@ function sendSubresource(subresourceExtractor) {
 
     authz.assertAuthorized(req.user, authz.actions.Read, subresource.resource);
 
-    const subresourceUrl = await subresource.url();
+    return await sendAny(req, res,
+      await subresource.url(),
+      subresource.accept
+    );
+  };
+}
 
-    /* Proxy the data through us:
-     *
-     *    client (browser, CLI, etc) ⟷ us (nextstrain.org) ⟷ upstream source
-     */
-    return await proxyResponseBodyFromUpstream(req, res, new Request(subresourceUrl, {
-      headers: {
-        Accept: subresource.accept,
-
-        /* Normalize Accept-Encoding to gzip or nothing, as realistically we
-         * only expect upstreams to support gzip and normalizing reduces the
-         * cache burden of varying on many equivalent but effectively the same
-         * client Accept-Encoding values.
-         */
-        "Accept-Encoding": req.acceptsEncodings("gzip")
-          ? "gzip"
-          : "identity",
-      },
-
-      /* Use "no-cache" mode to always revalidate with the upstream but avoid
-       * re-transferring the content if we have a cached copy that matches
-       * upstream.
-       */
-      cache: "no-cache",
 
-      /* Disable automatic body decompression.  Instead, we pass through the
-       * Accept-Encoding of the client to the upstream and return the
-       * upstream Content-Encoding and body directly.  This avoids needlessly
-       * decompressing and recompressing ourselves.
+/**
+ * Proxy the data through us:
+ *
+ *    client (browser, CLI, etc) ⟷ us (nextstrain.org) ⟷ upstream source
+ *
+ * @param {express.request} req - Express-style request instance
+ * @param {express.response} res - Express-style response instance
+ * @param {string} url - Resource URL
+ * @param {string} accept - HTTP Accept header value
+ * @returns {expressEndpointAsync}
+ */
+async function sendAny(req, res, url, accept) {
+  return await proxyResponseBodyFromUpstream(req, res, new Request(url, {
+    headers: {
+      Accept: accept,
+
+      /* Normalize Accept-Encoding to gzip or nothing, as realistically we
+       * only expect upstreams to support gzip and normalizing reduces the
+       * cache burden of varying on many equivalent but effectively the same
+       * client Accept-Encoding values.
        */
-      compress: false,
-    }));
-  };
+      "Accept-Encoding": req.acceptsEncodings("gzip")
+        ? "gzip"
+        : "identity",
+    },
+
+    /* Use "no-cache" mode to always revalidate with the upstream but avoid
+     * re-transferring the content if we have a cached copy that matches
+     * upstream.
+     */
+    cache: "no-cache",
+
+    /* Disable automatic body decompression.  Instead, we pass through the
+     * Accept-Encoding of the client to the upstream and return the
+     * upstream Content-Encoding and body directly.  This avoids needlessly
+     * decompressing and recompressing ourselves.
+     */
+    compress: false,
+  }));
 }