diff --git a/src/libstore/builtins/fetchurl.cc b/src/libstore/builtins/fetchurl.cc
index 4fb67f933..b9dfeba2f 100644
--- a/src/libstore/builtins/fetchurl.cc
+++ b/src/libstore/builtins/fetchurl.cc
@@ -23,9 +23,8 @@ void builtinFetchurl(
     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");
+    if (!(drv.type().isFixed() || drv.type().isImpure()))
+        throw Error("'builtin:fetchurl' must be a fixed-output or impure derivation");
 
     auto storePath = outputs.at("out");
     auto mainUrl = drv.env.at("url");
@@ -64,7 +63,8 @@ void builtinFetchurl(
     };
 
     /* Try the hashed mirrors first. */
-    if (dof->ca.method.getFileIngestionMethod() == FileIngestionMethod::Flat)
+    auto dof = std::get_if<DerivationOutput::CAFixed>(&out->raw);
+    if (dof && dof->ca.method.getFileIngestionMethod() == FileIngestionMethod::Flat)
         for (auto hashedMirror : settings.hashedMirrors.get())
             try {
                 if (!hasSuffix(hashedMirror, "/")) hashedMirror += '/';
diff --git a/tests/functional/fetchurl.sh b/tests/functional/fetchurl.sh
index 5a05cc5e1..a3620f52b 100644
--- a/tests/functional/fetchurl.sh
+++ b/tests/functional/fetchurl.sh
@@ -83,4 +83,4 @@ test -L $outPath/symlink
 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'
+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 or impure derivation'
diff --git a/tests/functional/impure-derivations.sh b/tests/functional/impure-derivations.sh
index 39d053a04..54ed6f5dd 100644
--- a/tests/functional/impure-derivations.sh
+++ b/tests/functional/impure-derivations.sh
@@ -63,3 +63,7 @@ path5=$(nix build -L --no-link --json --file ./impure-derivations.nix contentAdd
 path6=$(nix build -L --no-link --json --file ./impure-derivations.nix inputAddressedAfterCA | jq -r .[].outputs.out)
 [[ $(< $path6) = X ]]
 [[ $(< $TEST_ROOT/counter) = 5 ]]
+
+# Test nix/fetchurl.nix.
+path7=$(nix build -L --no-link --print-out-paths --expr "import <nix/fetchurl.nix> { impure = true; url = file://$PWD/impure-derivations.sh; }")
+cmp $path7 $PWD/impure-derivations.sh