diff --git a/src/Compilers/Core/Portable/Diagnostic/Diagnostic.cs b/src/Compilers/Core/Portable/Diagnostic/Diagnostic.cs
index 874a4777b717f754efd96cdf561f719985e592e9..37bc00aa5b8c8976cea8e0f33d867aee20119ce2 100644
--- a/src/Compilers/Core/Portable/Diagnostic/Diagnostic.cs
+++ b/src/Compilers/Core/Portable/Diagnostic/Diagnostic.cs
@@ -182,7 +182,7 @@ public abstract partial class Diagnostic : IEquatable<Diagnostic>, IFormattable
             var warningLevel = GetDefaultWarningLevel(effectiveSeverity, descriptor.DefaultSeverity);
             return SimpleDiagnostic.Create(
                 descriptor,
-                severity: effectiveSeverity.ToDiagnosticSeverity(descriptor.DefaultSeverity),
+                severity: effectiveSeverity.ToDiagnosticSeverity() ?? descriptor.DefaultSeverity,
                 warningLevel: warningLevel,
                 location: location ?? Location.None,
                 additionalLocations: additionalLocations,
diff --git a/src/Compilers/Core/Portable/Diagnostic/ReportDiagnosticExtensions.cs b/src/Compilers/Core/Portable/Diagnostic/ReportDiagnosticExtensions.cs
index c5da2e778b476ac5215b4625e3388cdc65e9bbcb..32b56e979f6096a70a579d4c63795f1001884c85 100644
--- a/src/Compilers/Core/Portable/Diagnostic/ReportDiagnosticExtensions.cs
+++ b/src/Compilers/Core/Portable/Diagnostic/ReportDiagnosticExtensions.cs
@@ -6,7 +6,7 @@ namespace Microsoft.CodeAnalysis
 {
     public static class ReportDiagnosticExtensions
     {
-        public static DiagnosticSeverity ToDiagnosticSeverity(this ReportDiagnostic reportDiagnostic, DiagnosticSeverity defaultSeverity)
+        public static DiagnosticSeverity? ToDiagnosticSeverity(this ReportDiagnostic reportDiagnostic)
         {
             switch (reportDiagnostic)
             {
@@ -24,7 +24,7 @@ public static DiagnosticSeverity ToDiagnosticSeverity(this ReportDiagnostic repo
 
             case ReportDiagnostic.Suppress:
             case ReportDiagnostic.Default:
-                return defaultSeverity;
+                return null;
 
             default:
                 throw ExceptionUtilities.UnexpectedValue(reportDiagnostic);
diff --git a/src/Compilers/Core/Portable/PublicAPI.Unshipped.txt b/src/Compilers/Core/Portable/PublicAPI.Unshipped.txt
index 72403fe09db9f90216a40ee6b14f76813286bb13..f83a9fc17fbd5324d884ff6ea0024bfac1a068a5 100644
--- a/src/Compilers/Core/Portable/PublicAPI.Unshipped.txt
+++ b/src/Compilers/Core/Portable/PublicAPI.Unshipped.txt
@@ -7,5 +7,5 @@ Microsoft.CodeAnalysis.ReportDiagnosticExtensions
 abstract Microsoft.CodeAnalysis.Compilation.ClassifyCommonConversion(Microsoft.CodeAnalysis.ITypeSymbol source, Microsoft.CodeAnalysis.ITypeSymbol destination) -> Microsoft.CodeAnalysis.Operations.CommonConversion
 static Microsoft.CodeAnalysis.Diagnostic.Create(Microsoft.CodeAnalysis.DiagnosticDescriptor descriptor, Microsoft.CodeAnalysis.Location location, Microsoft.CodeAnalysis.ReportDiagnostic effectiveSeverity, System.Collections.Generic.IEnumerable<Microsoft.CodeAnalysis.Location> additionalLocations, System.Collections.Immutable.ImmutableDictionary<string, string> properties, params object[] messageArgs) -> Microsoft.CodeAnalysis.Diagnostic
 static Microsoft.CodeAnalysis.DiagnosticSeverityExtensions.ToReportDiagnostic(this Microsoft.CodeAnalysis.DiagnosticSeverity diagnosticSeverity) -> Microsoft.CodeAnalysis.ReportDiagnostic
-static Microsoft.CodeAnalysis.ReportDiagnosticExtensions.ToDiagnosticSeverity(this Microsoft.CodeAnalysis.ReportDiagnostic reportDiagnostic, Microsoft.CodeAnalysis.DiagnosticSeverity defaultSeverity) -> Microsoft.CodeAnalysis.DiagnosticSeverity
+static Microsoft.CodeAnalysis.ReportDiagnosticExtensions.ToDiagnosticSeverity(this Microsoft.CodeAnalysis.ReportDiagnostic reportDiagnostic) -> Microsoft.CodeAnalysis.DiagnosticSeverity?
 static Microsoft.CodeAnalysis.ReportDiagnosticExtensions.WithDefaultSeverity(this Microsoft.CodeAnalysis.ReportDiagnostic reportDiagnostic, Microsoft.CodeAnalysis.DiagnosticSeverity defaultSeverity) -> Microsoft.CodeAnalysis.ReportDiagnostic
diff --git a/src/Workspaces/Core/Portable/CodeStyle/CodeStyleOption.cs b/src/Workspaces/Core/Portable/CodeStyle/CodeStyleOption.cs
index 6fe565e4696c5de2fe53d26185e334f2955bcabc..5ceeae8559b43d956e6fba095d72422c45606803 100644
--- a/src/Workspaces/Core/Portable/CodeStyle/CodeStyleOption.cs
+++ b/src/Workspaces/Core/Portable/CodeStyle/CodeStyleOption.cs
@@ -49,7 +49,7 @@ public CodeStyleOption(T value, NotificationOption notification)
                 new XAttribute(nameof(SerializationVersion), SerializationVersion),
                 new XAttribute("Type", GetTypeNameForSerialization()),
                 new XAttribute(nameof(Value), GetValueForSerialization()),
-                new XAttribute(nameof(DiagnosticSeverity), Notification.Severity.ToDiagnosticSeverity(DiagnosticSeverity.Hidden)));
+                new XAttribute(nameof(DiagnosticSeverity), Notification.Severity.ToDiagnosticSeverity() ?? DiagnosticSeverity.Hidden));
 
         private object GetValueForSerialization()
         {
diff --git a/src/Workspaces/Core/Portable/CodeStyle/NotificationOption.cs b/src/Workspaces/Core/Portable/CodeStyle/NotificationOption.cs
index 93cb8e3615bcc1de9932c8a239461d2e70b1ed5e..fa2753192044ee76f9344b3e06d33b6f0f2ce017 100644
--- a/src/Workspaces/Core/Portable/CodeStyle/NotificationOption.cs
+++ b/src/Workspaces/Core/Portable/CodeStyle/NotificationOption.cs
@@ -25,7 +25,7 @@ public ReportDiagnostic Severity
         [Obsolete("Use " + nameof(Severity) + " instead.")]
         public DiagnosticSeverity Value
         {
-            get => Severity.ToDiagnosticSeverity(DiagnosticSeverity.Hidden);
+            get => Severity.ToDiagnosticSeverity() ?? DiagnosticSeverity.Hidden;
             set => Severity = value.ToReportDiagnostic();
         }
 
diff --git a/src/Workspaces/Core/Portable/NamingStyles/Serialization/SerializableNamingRule.cs b/src/Workspaces/Core/Portable/NamingStyles/Serialization/SerializableNamingRule.cs
index e71ab30491054e91d577fa1920a34ccfc68db65b..01896985dc7e7e5a254b032d1d3e4a34aef4e670 100644
--- a/src/Workspaces/Core/Portable/NamingStyles/Serialization/SerializableNamingRule.cs
+++ b/src/Workspaces/Core/Portable/NamingStyles/Serialization/SerializableNamingRule.cs
@@ -24,7 +24,7 @@ internal XElement CreateXElement()
             var element = new XElement(nameof(SerializableNamingRule),
                 new XAttribute(nameof(SymbolSpecificationID), SymbolSpecificationID),
                 new XAttribute(nameof(NamingStyleID), NamingStyleID),
-                new XAttribute(nameof(EnforcementLevel), EnforcementLevel.ToDiagnosticSeverity(DiagnosticSeverity.Hidden)));
+                new XAttribute(nameof(EnforcementLevel), EnforcementLevel.ToDiagnosticSeverity() ?? DiagnosticSeverity.Hidden));
 
             return element;
         }