diff --git a/ILSpy.BamlDecompiler.Tests/Cases/NamespacePrefix.xaml b/ILSpy.BamlDecompiler.Tests/Cases/NamespacePrefix.xaml
index e8991e7da..a147588b2 100644
--- a/ILSpy.BamlDecompiler.Tests/Cases/NamespacePrefix.xaml
+++ b/ILSpy.BamlDecompiler.Tests/Cases/NamespacePrefix.xaml
@@ -1,4 +1,4 @@
-<Grid xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation" xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml" xmlns:cc="clr-namespace:ILSpy.BamlDecompiler.Tests.Cases">
+<Grid xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation" xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml" xmlns:cc="clr-namespace:ILSpy.BamlDecompiler.Tests.Cases" xmlns:unk="clr-namespace:A;assembly=Unknown">
 	<cc:CustomControl Grid.Row="0" Tag="{}{}{Test}" CustomName="Custom1">
 		<FrameworkElement.Style>
 			<Style />
diff --git a/ILSpy.BamlDecompiler/Xaml/NamespaceMap.cs b/ILSpy.BamlDecompiler/Xaml/NamespaceMap.cs
index 4ca225fcb..55c384304 100644
--- a/ILSpy.BamlDecompiler/Xaml/NamespaceMap.cs
+++ b/ILSpy.BamlDecompiler/Xaml/NamespaceMap.cs
@@ -36,11 +36,11 @@ namespace ILSpy.BamlDecompiler.Xaml {
 
 		public NamespaceMap(string prefix, IModule asm, string xmlNs, string clrNs) {
 			XmlnsPrefix = prefix;
-			Assembly = asm ?? throw new ArgumentNullException(nameof(asm));
+			Assembly = asm;
 			XMLNamespace = xmlNs;
 			CLRNamespace = clrNs;
 		}
 
-		public override string ToString() => $"{XmlnsPrefix}:[{Assembly.Name}|{CLRNamespace ?? XMLNamespace}]";
+		public override string ToString() => $"{XmlnsPrefix}:[{Assembly?.Name}|{CLRNamespace ?? XMLNamespace}]";
 	}
 }
\ No newline at end of file
diff --git a/ILSpy.BamlDecompiler/XmlnsDictionary.cs b/ILSpy.BamlDecompiler/XmlnsDictionary.cs
index bb75fe687..d4f91d8bf 100644
--- a/ILSpy.BamlDecompiler/XmlnsDictionary.cs
+++ b/ILSpy.BamlDecompiler/XmlnsDictionary.cs
@@ -37,7 +37,7 @@ namespace ILSpy.BamlDecompiler {
 
 		public string LookupXmlns(IModule asm, string clrNs) {
 			foreach (var ns in this) {
-				if (asm.FullAssemblyName == ns.Assembly.FullAssemblyName && ns.CLRNamespace == clrNs)
+				if (asm.FullAssemblyName == ns.Assembly?.FullAssemblyName && ns.CLRNamespace == clrNs)
 					return ns.XMLNamespace;
 			}
 
@@ -103,14 +103,14 @@ namespace ILSpy.BamlDecompiler {
 
 		public string LookupXmlns(IModule asm, string clrNs) {
 			foreach (var map in piMappings) {
-				if (asm.FullAssemblyName == map.Value.Assembly.FullAssemblyName && map.Value.CLRNamespace == clrNs)
+				if (asm.FullAssemblyName == map.Value.Assembly?.FullAssemblyName && map.Value.CLRNamespace == clrNs)
 					return map.Key;
 			}
 
 			var scope = CurrentScope;
 			while (scope != null) {
 				foreach (var ns in scope) {
-					if (asm.FullAssemblyName == ns.Assembly.FullAssemblyName && ns.CLRNamespace == clrNs)
+					if (asm.FullAssemblyName == ns.Assembly?.FullAssemblyName && ns.CLRNamespace == clrNs)
 						return ns.XMLNamespace;
 				}