diff --git a/src/nix-store/nix-store.cc b/src/nix-store/nix-store.cc
index 4fee7258cb..5bcb82f324 100644
--- a/src/nix-store/nix-store.cc
+++ b/src/nix-store/nix-store.cc
@@ -923,7 +923,7 @@ static void opServe(Strings opFlags, Strings opArgs)
             }
             break;
         case cmdSubstitute:
-            dumpPath(readString(in), out);
+            dumpPath(readStorePath(in), out);
             break;
         default:
             throw Error(format("unknown serve command `%1%'") % cmd);