diff --git a/pkgs/build-support/trivial-builders/default.nix b/pkgs/build-support/trivial-builders/default.nix
index d095974513cb..dcdac09004bb 100644
--- a/pkgs/build-support/trivial-builders/default.nix
+++ b/pkgs/build-support/trivial-builders/default.nix
@@ -824,9 +824,10 @@ rec {
         or
           nix-prefetch-url --type ${hashAlgo} file:///path/to/${name_}
       '';
-      hashAlgo = if hash != null then ""
+      hashAlgo = if hash != null then (builtins.head (lib.strings.splitString "-" hash))
             else if sha256 != null then "sha256"
             else "sha1";
+      hashAlgo_ = if hash != null then "" else hashAlgo;
       hash_ = if hash != null then hash
          else if sha256 != null then sha256
          else sha1;
@@ -835,7 +836,7 @@ rec {
     stdenvNoCC.mkDerivation {
       name = name_;
       outputHashMode = hashMode;
-      outputHashAlgo = hashAlgo;
+      outputHashAlgo = hashAlgo_;
       outputHash = hash_;
       preferLocalBuild = true;
       allowSubstitutes = false;