diff --git a/lib/cabal-project-parser.nix b/lib/cabal-project-parser.nix
index 01672b90b9..0d93ba157b 100644
--- a/lib/cabal-project-parser.nix
+++ b/lib/cabal-project-parser.nix
@@ -74,9 +74,15 @@ let
   #   --shar256: 003lm3pm0000hbfmii7xcdd9v20000flxf7gdl2pyxia7p014i8z
   # will be trated like a field and returned here
   # (used in call-cabal-project-to-nix.nix to create a fixed-output derivation)
-  extractSourceRepoPackageData = cabalProjectFileName: sha256map: repo: {
+  extractSourceRepoPackageData = cabalProjectFileName: sha256map: repo:
+    let
+      refOrRev =
+        if builtins.match "[0-9a-f](40)" repo.tag != null
+          then "rev"
+          else "ref";
+    in {
     url = repo.location;
-    ref = repo.tag;
+    "${refOrRev}" = repo.tag;
     sha256 = repo."--sha256" or (
       if sha256map != null
         then sha256map."${repo.location}"."${repo.tag}"
diff --git a/lib/call-cabal-project-to-nix.nix b/lib/call-cabal-project-to-nix.nix
index 1517574075..7ee2016dea 100644
--- a/lib/call-cabal-project-to-nix.nix
+++ b/lib/call-cabal-project-to-nix.nix
@@ -228,10 +228,10 @@ let
                   then throw "${inputMap.${repoData.url}.rev} may not match ${repoData.ref} for ${repoData.url} use \"${repoData.url}/${repoData.ref}\" as the inputMap key if ${repoData.ref} is a branch or tag that points to ${inputMap.${repoData.url}.rev}."
                   else inputMap.${repoData.url})
             else if repoData.sha256 != null
-              then fetchgit { inherit (repoData) url sha256; rev = repoData.ref; }
+            then fetchgit { inherit (repoData) url sha256; rev = repoData.rev or repoData.ref; }
             else
-              let drv = builtins.fetchGit { inherit (repoData) url ref; };
-              in __trace "WARNING: No sha256 found for source-repository-package ${repoData.url} ${repoData.ref} download may fail in restricted mode (hydra)"
+              let drv = builtins.fetchGit { inherit (repoData) url ; rev = repoData.rev or repoData.ref; ref = repoData.ref or null; };
+              in __trace "WARNING: No sha256 found for source-repository-package ${repoData.url} ref=${repoData.ref or "(unspecified)"} rev=${repoData.rev or "(unspecified)"} download may fail in restricted mode (hydra)"
                 (__trace "Consider adding `--sha256: ${hashPath drv}` to the ${cabalProjectFileName} file or passing in a sha256map argument"
                  drv);
         in {