diff --git a/pkgs/build-support/fetchhg/default.nix b/pkgs/build-support/fetchhg/default.nix
index 7f4ed2ff3e8c..d53e39699d35 100644
--- a/pkgs/build-support/fetchhg/default.nix
+++ b/pkgs/build-support/fetchhg/default.nix
@@ -30,14 +30,18 @@ lib.extendMkDerivation {
 
       outputHashAlgo = if finalAttrs.hash != null && finalAttrs.hash != "" then null else "sha256";
       outputHashMode = "recursive";
-      outputHash = lib.throwIf (hash != null && sha256 != null) "Only one of sha256 or hash can be set" (
-        if finalAttrs.hash != null then
-          finalAttrs.hash
-        else if sha256 != null then
-          sha256
+      outputHash =
+        if (hash != null && sha256 != null) then
+          throw "Only one of sha256 or hash can be set"
         else
-          ""
-      );
+          (
+            if finalAttrs.hash != null then
+              finalAttrs.hash
+            else if sha256 != null then
+              sha256
+            else
+              ""
+          );
 
       inherit url rev hash;
       inherit preferLocalBuild;