diff --git a/src/libstore/builtins/fetchurl.cc b/src/libstore/builtins/fetchurl.cc
index 831431437..770df2927 100644
--- a/src/libstore/builtins/fetchurl.cc
+++ b/src/libstore/builtins/fetchurl.cc
@@ -63,7 +63,7 @@ void builtinFetchurl(const BasicDerivation & drv, const std::string & netrcData)
         for (auto hashedMirror : settings.hashedMirrors.get())
             try {
                 if (!hasSuffix(hashedMirror, "/")) hashedMirror += '/';
-                auto ht = parseHashType(getAttr("outputHashAlgo"));
+                auto ht = parseHashTypeOpt(getAttr("outputHashAlgo"));
                 auto h = Hash(getAttr("outputHash"), ht);
                 fetch(hashedMirror + printHashType(*h.type) + "/" + h.to_string(Base::Base16, false));
                 return;