diff --git a/src/EditorFeatures/CSharpTest/UsePatternMatching/CSharpAsAndNullCheckTests.cs b/src/EditorFeatures/CSharpTest/UsePatternMatching/CSharpAsAndNullCheckTests.cs
index f514b7cbe0b0793ee260c764d7e6c519c5e59ccb..6170954f319e071c750faf62f771628cddd53627 100644
--- a/src/EditorFeatures/CSharpTest/UsePatternMatching/CSharpAsAndNullCheckTests.cs
+++ b/src/EditorFeatures/CSharpTest/UsePatternMatching/CSharpAsAndNullCheckTests.cs
@@ -726,6 +726,23 @@ void M()
         {
         }
     }
+}");
+        }
+
+        [WorkItem(21172, "https://github.com/dotnet/roslyn/issues/21172")]
+        [Fact, Trait(Traits.Feature, Traits.Features.CodeActionsInlineTypeCheck)]
+        public async Task TestMissingWithDynamic()
+        {
+            await TestMissingAsync(
+@"class C
+{
+    void M(object o)
+    {
+        [|var|] x = o as dynamic;
+        if (x != null)
+        {
+        }
+    }
 }");
         }
     }
diff --git a/src/EditorFeatures/CSharpTest/UsePatternMatching/CSharpIsAndCastCheckDiagnosticAnalyzerTests.cs b/src/EditorFeatures/CSharpTest/UsePatternMatching/CSharpIsAndCastCheckDiagnosticAnalyzerTests.cs
index 7f16164a55d6bb01df2f7bdc0a7fbf2c34db0fc0..90a9d1d5be3c9976f75726f4f6b5e050c2fa4b55 100644
--- a/src/EditorFeatures/CSharpTest/UsePatternMatching/CSharpIsAndCastCheckDiagnosticAnalyzerTests.cs
+++ b/src/EditorFeatures/CSharpTest/UsePatternMatching/CSharpIsAndCastCheckDiagnosticAnalyzerTests.cs
@@ -538,5 +538,25 @@ public object Convert(object value)
     }
 }");
         }
+
+        [WorkItem(21172, "https://github.com/dotnet/roslyn/issues/21172")]
+        [Fact, Trait(Traits.Feature, Traits.Features.CodeActionsInlineTypeCheck)]
+        public async Task TestMissingWithDynamic()
+        {
+            await TestMissingInRegularAndScriptAsync(
+@"
+class C
+{
+    public object Convert(object value)
+    {
+        if (value is dynamic)
+        {
+            [|dynamic|] tmp = (dynamic)value;
+        }
+
+        return null;
+    }
+}");
+        }
     }
 }
diff --git a/src/Features/CSharp/Portable/UsePatternMatching/CSharpAsAndNullCheckDiagnosticAnalyzer.cs b/src/Features/CSharp/Portable/UsePatternMatching/CSharpAsAndNullCheckDiagnosticAnalyzer.cs
index 33733171a2dbbd43280e266ddd0b79e60a0b61f0..2a8fe989e9a61f0f8bc4aa431313e2d98c110570 100644
--- a/src/Features/CSharp/Portable/UsePatternMatching/CSharpAsAndNullCheckDiagnosticAnalyzer.cs
+++ b/src/Features/CSharp/Portable/UsePatternMatching/CSharpAsAndNullCheckDiagnosticAnalyzer.cs
@@ -103,6 +103,12 @@ private void SyntaxNodeAction(SyntaxNodeAnalysisContext syntaxContext)
                 return;
             }
 
+            if (asType?.TypeKind == TypeKind.Dynamic)
+            {
+                // Not legal to use dynamic in a pattern.
+                return;
+            }
+
             var localSymbol = (ILocalSymbol)semanticModel.GetDeclaredSymbol(declarator);
             if (!localSymbol.Type.Equals(asType))
             {
diff --git a/src/Features/CSharp/Portable/UsePatternMatching/CSharpIsAndCastCheckDiagnosticAnalyzer.cs b/src/Features/CSharp/Portable/UsePatternMatching/CSharpIsAndCastCheckDiagnosticAnalyzer.cs
index 1428762ef9a679bd7bcc047a2b06ea5c9e46c711..1d8249673bd5717d91d7be927ee2de94f7dd37f1 100644
--- a/src/Features/CSharp/Portable/UsePatternMatching/CSharpIsAndCastCheckDiagnosticAnalyzer.cs
+++ b/src/Features/CSharp/Portable/UsePatternMatching/CSharpIsAndCastCheckDiagnosticAnalyzer.cs
@@ -112,6 +112,12 @@ private void SyntaxNodeAction(SyntaxNodeAnalysisContext syntaxContext)
                 return;
             }
 
+            if (isType?.TypeKind == TypeKind.Dynamic)
+            {
+                // Not legal to use dynamic in a pattern.
+                return;
+            }
+
             if (!localSymbol.Type.Equals(isType))
             {
                 // we have something like: