diff --git a/HostsFileEditor.Core.Tests/HostsEntryTests.cs b/HostsFileEditor.Core.Tests/HostsEntryTests.cs
index 9aaf604..d52cc67 100644
--- a/HostsFileEditor.Core.Tests/HostsEntryTests.cs
+++ b/HostsFileEditor.Core.Tests/HostsEntryTests.cs
@@ -81,6 +81,20 @@ public void Invalid_Hostname_AfterEdit()
         entry.Valid.ShouldBeFalse();
     }
 
+    // Regression for #131: Resources.InvalidHostnames had no matching key in Core's Resources.resx,
+    // so it resolved to null and SetError(null) cleared the error — an invalid hostname surfaced a
+    // BLANK IDataErrorInfo message (rather than "Invalid host names") in both editions.
+    [TestMethod]
+    public void Invalid_Hostname_HasNonBlankErrorMessage()
+    {
+        var entry = new HostsEntry("127.0.0.1 goodhost");
+        entry.HostNames = "bad@host"; // invalid character
+
+        entry.Valid.ShouldBeFalse();
+        entry["HostNames"].ShouldNotBeNullOrEmpty();
+        entry["HostNames"].ShouldBe(HostsFileEditor.Properties.Resources.InvalidHostnames);
+    }
+
     [TestMethod]
     public void ToString_ReturnsExpected()
     {
diff --git a/HostsFileEditor.Core/Properties/Resources.resx b/HostsFileEditor.Core/Properties/Resources.resx
index 039c016..d5114da 100644
--- a/HostsFileEditor.Core/Properties/Resources.resx
+++ b/HostsFileEditor.Core/Properties/Resources.resx
@@ -68,6 +68,9 @@
   <data name="InvalidHostEntries" xml:space="preserve">
     <value>Invalid Host Entries</value>
   </data>
+  <data name="InvalidHostnames" xml:space="preserve">
+    <value>Invalid host names</value>
+  </data>
   <data name="LoseChangesDialogCaption" xml:space="preserve">
     <value>Hosts File Editor Question</value>
   </data>