diff --git a/csharp/PhoneNumbers.Extensions.Test/TestPhoneNumberTypeConverter.cs b/csharp/PhoneNumbers.Extensions.Test/TestPhoneNumberTypeConverter.cs
index aa0da2ae..fbdf4235 100644
--- a/csharp/PhoneNumbers.Extensions.Test/TestPhoneNumberTypeConverter.cs
+++ b/csharp/PhoneNumbers.Extensions.Test/TestPhoneNumberTypeConverter.cs
@@ -1,3 +1,4 @@
+using System;
 using Xunit;
 
 namespace PhoneNumbers.Extensions.Test
@@ -15,6 +16,14 @@ public void ConvertFrom_ParsesString()
             Assert.Equal(6192987704UL, number.NationalNumber);
         }
 
+        [Fact]
+        public void ConvertFrom_InvalidString_ThrowsFormatException()
+        {
+            var ex = Assert.Throws<FormatException>(() => Converter.ConvertFrom("not-a-number"));
+
+            Assert.IsType<NumberParseException>(ex.InnerException);
+        }
+
         [Fact]
         public void ConvertFrom_MatchesTryParse_ForEqualityAndHashing()
         {
diff --git a/csharp/PhoneNumbers.Extensions/PhoneNumberTypeConverter.cs b/csharp/PhoneNumbers.Extensions/PhoneNumberTypeConverter.cs
index 920e909d..4aa571ed 100644
--- a/csharp/PhoneNumbers.Extensions/PhoneNumberTypeConverter.cs
+++ b/csharp/PhoneNumbers.Extensions/PhoneNumberTypeConverter.cs
@@ -17,9 +17,21 @@ public override bool CanConvertFrom(ITypeDescriptorContext context, Type sourceT
             => sourceType == typeof(string) || base.CanConvertFrom(context, sourceType);
 
         public override object ConvertFrom(ITypeDescriptorContext context, CultureInfo culture, object value)
-            => value is string stringValue
-                ? Util.Parse(stringValue, null)
-                : base.ConvertFrom(context, culture, value);
+        {
+            if (value is not string stringValue)
+            {
+                return base.ConvertFrom(context, culture, value);
+            }
+
+            try
+            {
+                return Util.Parse(stringValue, null);
+            }
+            catch (NumberParseException ex)
+            {
+                throw new FormatException($"'{stringValue}' is not a valid phone number.", ex);
+            }
+        }
 
         public override bool CanConvertTo(ITypeDescriptorContext context, Type destinationType)
             => destinationType == typeof(string) || base.CanConvertTo(context, destinationType);
diff --git a/csharp/PhoneNumbers.MetadataBuilder/Program.cs b/csharp/PhoneNumbers.MetadataBuilder/Program.cs
index a51312c0..de33c088 100644
--- a/csharp/PhoneNumbers.MetadataBuilder/Program.cs
+++ b/csharp/PhoneNumbers.MetadataBuilder/Program.cs
@@ -189,7 +189,7 @@ private static int BuildLocaleNames(string inputFile, string outputDir)
         var byCountry = ParseLocaleText(inputFile);
         foreach (var country in byCountry)
         {
-            var outPath = Path.Join(outputDir, country.Key);
+            var outPath = Path.Join(outputDir, Path.GetFileName(country.Key));
             using var gz = new GZipStream(File.Create(outPath), CompressionLevel.SmallestSize);
             BuildPrefixMapFromBin.WriteLocaleNames(gz, country.Value);
         }