diff --git a/src/libstore/builtins/fetchurl.cc b/src/libstore/builtins/fetchurl.cc
index 2086bd0b9..a9f2e748e 100644
--- a/src/libstore/builtins/fetchurl.cc
+++ b/src/libstore/builtins/fetchurl.cc
@@ -16,6 +16,14 @@ void builtinFetchurl(const BasicDerivation & drv, const std::string & netrcData)
         writeFile(settings.netrcFile, netrcData, 0600);
     }
 
+    auto out = get(drv.outputs, "out");
+    if (!out)
+        throw Error("'builtin:fetchurl' requires an 'out' output");
+
+    auto dof = std::get_if<DerivationOutput::CAFixed>(&out->raw);
+    if (!dof)
+        throw Error("'builtin:fetchurl' must be a fixed-output derivation");
+
     auto getAttr = [&](const std::string & name) {
         auto i = drv.env.find(name);
         if (i == drv.env.end()) throw Error("attribute '%s' missing", name);
@@ -59,13 +67,11 @@ void builtinFetchurl(const BasicDerivation & drv, const std::string & netrcData)
     };
 
     /* Try the hashed mirrors first. */
-    if (getAttr("outputHashMode") == "flat")
+    if (dof->ca.method.getFileIngestionMethod() == FileIngestionMethod::Flat)
         for (auto hashedMirror : settings.hashedMirrors.get())
             try {
                 if (!hasSuffix(hashedMirror, "/")) hashedMirror += '/';
-                std::optional<HashAlgorithm> ht = parseHashAlgoOpt(getAttr("outputHashAlgo"));
-                Hash h = newHashAllowEmpty(getAttr("outputHash"), ht);
-                fetch(hashedMirror + printHashAlgo(h.algo) + "/" + h.to_string(HashFormat::Base16, false));
+                fetch(hashedMirror + printHashAlgo(dof->ca.hash.algo) + "/" + dof->ca.hash.to_string(HashFormat::Base16, false));
                 return;
             } catch (Error & e) {
                 debug(e.what());
diff --git a/tests/functional/fetchurl.sh b/tests/functional/fetchurl.sh
index 8cd40c09f..5a05cc5e1 100644
--- a/tests/functional/fetchurl.sh
+++ b/tests/functional/fetchurl.sh
@@ -78,3 +78,9 @@ outPath=$(nix-build -vvvvv --expr 'import <nix/fetchurl.nix>' --argstr url file:
 
 test -x $outPath/fetchurl.sh
 test -L $outPath/symlink
+
+# Make sure that *not* passing a outputHash fails.
+requireDaemonNewerThan "2.20"
+expected=100
+if [[ -v NIX_DAEMON_PACKAGE ]]; then expected=1; fi # work around the daemon not returning a 100 status correctly
+expectStderr $expected nix-build --expr '{ url }: builtins.derivation { name = "nix-cache-info"; system = "x86_64-linux"; builder = "builtin:fetchurl"; inherit url; outputHashMode = "flat"; }' --argstr url file://$narxz 2>&1 | grep 'must be a fixed-output derivation'