diff --git a/src/Att.cpp b/src/Att.cpp
index 312c91f36..29847ebf2 100644
--- a/src/Att.cpp
+++ b/src/Att.cpp
@@ -107,7 +107,12 @@ bool Att::parse (Nibbler& n)
     {
       std::string mod;
       if (n.getUntilOneOf (".:", mod))
-        mMods.push_back (mod);
+      {
+        if (validMod (mod))
+          mMods.push_back (mod);
+        else
+          throw std::string ("The name '") + mod + "' is not a valid modifier";
+      }
       else
         throw std::string ("Missing . or : after modifier");
     }
@@ -212,7 +217,10 @@ std::string Att::composeF4 () const
 ////////////////////////////////////////////////////////////////////////////////
 void Att::addMod (const std::string& mod)
 {
-  mMods.push_back (mod);
+  if (validMod (mod))
+    mMods.push_back (mod);
+  else
+    throw std::string ("The name '") + mod + "' is not a valid modifier";
 }
 
 ////////////////////////////////////////////////////////////////////////////////
diff --git a/src/tests/att.t.cpp b/src/tests/att.t.cpp
index 88f1f4b59..3ee651195 100644
--- a/src/tests/att.t.cpp
+++ b/src/tests/att.t.cpp
@@ -140,11 +140,6 @@ int main (int argc, char** argv)
   try {a6.addMod ("endswith");} catch (...) {good = false;}
   t.ok (good, "Att::addMod (endswith)");
 
-
-
-
-
-
   good = true;
   try {a6.addMod ("fartwizzle");} catch (...) {good = false;}
   t.notok (good, "Att::addMod (fartwizzle)");
@@ -152,8 +147,8 @@ int main (int argc, char** argv)
   // Att::mods
   std::vector <std::string> mods;
   a6.mods (mods);
-  t.is (mods.size (), (size_t)1, "Att::mods () size == 1");
-  t.is (mods[0], "is",           "Att::mods [0] == 'is'");
+  t.is (mods.size (), (size_t)18, "Att::mods () size == 18");
+  t.is (mods[0], "is",            "Att::mods [0] == 'is'");
 
   // Att::parse
   Nibbler n ("");