diff --git a/src/libutil/args.hh b/src/libutil/args.hh
index 6ed541a32..3e84ac64a 100644
--- a/src/libutil/args.hh
+++ b/src/libutil/args.hh
@@ -68,8 +68,12 @@ protected:
             , arity(ArityAny)
         { }
 
-        template<class T>
-        Handler(T * dest)
+        Handler(std::string * dest)
+            : fun([=](std::vector<std::string> ss) { *dest = ss[0]; })
+            , arity(1)
+        { }
+
+        Handler(std::optional<std::string> * dest)
             : fun([=](std::vector<std::string> ss) { *dest = ss[0]; })
             , arity(1)
         { }
@@ -79,6 +83,15 @@ protected:
             : fun([=](std::vector<std::string> ss) { *dest = val; })
             , arity(0)
         { }
+
+        template<class I>
+        Handler(I * dest)
+            : fun([=](std::vector<std::string> ss) {
+                if (!string2Int(ss[0], *dest))
+                    throw UsageError("'%s' is not an integer", ss[0]);
+              })
+            , arity(1)
+        { }
     };
 
     /* Flags. */
@@ -161,15 +174,6 @@ public:
         });
     }
 
-    template<class I>
-    void mkIntFlag(char shortName, const std::string & longName,
-        const std::string & description, I * dest)
-    {
-        mkFlag<I>(shortName, longName, description, [=](I n) {
-            *dest = n;
-        });
-    }
-
     template<class I>
     void mkFlag(char shortName, const std::string & longName,
         const std::string & description, std::function<void(I)> fun)
diff --git a/src/nix/verify.cc b/src/nix/verify.cc
index 16d42349f..620109aac 100644
--- a/src/nix/verify.cc
+++ b/src/nix/verify.cc
@@ -20,6 +20,7 @@ struct CmdVerify : StorePathsCommand
     {
         mkFlag(0, "no-contents", "do not verify the contents of each store path", &noContents);
         mkFlag(0, "no-trust", "do not verify whether each store path is trusted", &noTrust);
+
         addFlag({
             .longName = "substituter",
             .shortName = 's',
@@ -27,7 +28,14 @@ struct CmdVerify : StorePathsCommand
             .labels = {"store-uri"},
             .handler = {[&](std::string s) { substituterUris.push_back(s); }}
         });
-        mkIntFlag('n', "sigs-needed", "require that each path has at least N valid signatures", &sigsNeeded);
+
+        addFlag({
+            .longName = "sigs-needed",
+            .shortName = 'n',
+            .description = "require that each path has at least N valid signatures",
+            .labels = {"n"},
+            .handler = {&sigsNeeded}
+        });
     }
 
     std::string description() override