2 files changed, 2 insertions, 2 deletions

diff --git a/src/wixext/BalBurnBackendExtension.cs b/src/wixext/BalBurnBackendExtension.cs
index 6e639c1b..e8dc7a3e 100644
--- a/src/wixext/BalBurnBackendExtension.cs
+++ b/src/wixext/BalBurnBackendExtension.cs
@@ -25,7 +25,7 @@ namespace WixToolset.Bal
             BalSymbolDefinitions.WixStdbaOverridableVariable,
         };
 
-        protected override IEnumerable<IntermediateSymbolDefinition> SymbolDefinitions => BurnSymbolDefinitions;
+        protected override IReadOnlyCollection<IntermediateSymbolDefinition> SymbolDefinitions => BurnSymbolDefinitions;
 
         public override void SymbolsFinalized(IntermediateSection section)
         {
diff --git a/src/wixext/BalExtensionFactory.cs b/src/wixext/BalExtensionFactory.cs
index 30ec88c3..0bfb6c5f 100644
--- a/src/wixext/BalExtensionFactory.cs
+++ b/src/wixext/BalExtensionFactory.cs
@@ -8,7 +8,7 @@ namespace WixToolset.Bal
 
     public class BalExtensionFactory : BaseExtensionFactory
     {
-        protected override IEnumerable<Type> ExtensionTypes => new[]
+        protected override IReadOnlyCollection<Type> ExtensionTypes => new[]
         {
             typeof(BalCompiler),
             typeof(BalExtensionData),