diff --git a/nix/libstore/build.cc b/nix/libstore/build.cc
index 06bc7601b9..fe7bf79069 100644
--- a/nix/libstore/build.cc
+++ b/nix/libstore/build.cc
@@ -2348,7 +2348,7 @@ PathSet parseReferenceSpecifiers(const Derivation & drv, string attr)
         else if (drv.outputs.find(*i) != drv.outputs.end())
             result.insert(drv.outputs.find(*i)->second.path);
         else throw BuildError(
-            format("derivation contains an illegal reference specifier `%1%'")
+            format("derivation contains an invalid reference specifier `%1%'")
             % *i);
     }
     return result;
diff --git a/nix/libstore/globals.cc b/nix/libstore/globals.cc
index 69f6d06563..6df20e7a52 100644
--- a/nix/libstore/globals.cc
+++ b/nix/libstore/globals.cc
@@ -196,7 +196,7 @@ string Settings::pack()
         if (i->first.find('\n') != string::npos ||
             i->first.find('=') != string::npos ||
             i->second.find('\n') != string::npos)
-            throw Error("illegal option name/value");
+            throw Error("invalid option name/value");
         s += i->first; s += '='; s += i->second; s += '\n';
     }
     return s;
diff --git a/nix/libstore/store-api.cc b/nix/libstore/store-api.cc
index 709d17ea21..0de0b6b298 100644
--- a/nix/libstore/store-api.cc
+++ b/nix/libstore/store-api.cc
@@ -61,7 +61,7 @@ void checkStoreName(const string & name)
     /* Disallow names starting with a dot for possible security
        reasons (e.g., "." and ".."). */
     if (string(name, 0, 1) == ".")
-        throw Error(format("illegal name: `%1%'") % name);
+        throw Error(format("invalid name: `%1%'") % name);
     foreach (string::const_iterator, i, name)
         if (!((*i >= 'A' && *i <= 'Z') ||
               (*i >= 'a' && *i <= 'z') ||