diff --git a/src/nix-env/nix-env.cc b/src/nix-env/nix-env.cc
index 0da31588d..f6a9d9e7b 100644
--- a/src/nix-env/nix-env.cc
+++ b/src/nix-env/nix-env.cc
@@ -1287,7 +1287,7 @@ static void opDeleteGenerations(Globals & globals, Strings opFlags, Strings opAr
     } else if (opArgs.size() == 1 && opArgs.front().find('+') != string::npos) {
         if(opArgs.front().size() < 2)
             throw Error(format("invalid number of generations ‘%1%’") % opArgs.front());
-        string str_max = string(opArgs.front() 1, opArgs.front().size());
+        string str_max = string(opArgs.front(), 1, opArgs.front().size());
         int max;
         if (!string2Int(str_max, max) || max == 0)
             throw Error(format("invalid number of generations to keep ‘%1%’") % opArgs.front());