diff --git a/src/OmniSharp.Cake/CachingScriptMetadataResolver.cs b/src/OmniSharp.Cake/CachingScriptMetadataResolver.cs
index 67f2774891..583df1993f 100644
--- a/src/OmniSharp.Cake/CachingScriptMetadataResolver.cs
+++ b/src/OmniSharp.Cake/CachingScriptMetadataResolver.cs
@@ -1,4 +1,5 @@
-using System.Collections.Generic;
+using System.Collections.Concurrent;
+using System.Collections.Generic;
 using System.Collections.Immutable;
 using Microsoft.CodeAnalysis;
 using Microsoft.CodeAnalysis.Scripting;
@@ -7,8 +8,8 @@ namespace OmniSharp.Cake
 {
     internal class CachingScriptMetadataResolver : MetadataReferenceResolver
     {
-        private static readonly Dictionary<string, ImmutableArray<PortableExecutableReference>> DirectReferenceCache = new Dictionary<string, ImmutableArray<PortableExecutableReference>>();
-        private static readonly Dictionary<string, PortableExecutableReference> MissingReferenceCache = new Dictionary<string, PortableExecutableReference>();
+        private static readonly ConcurrentDictionary<string, ImmutableArray<PortableExecutableReference>> DirectReferenceCache = new ConcurrentDictionary<string, ImmutableArray<PortableExecutableReference>>();
+        private static readonly ConcurrentDictionary<string, PortableExecutableReference> MissingReferenceCache = new ConcurrentDictionary<string, PortableExecutableReference>();
         private static readonly MetadataReferenceResolver DefaultRuntimeResolver = ScriptMetadataResolver.Default;
 
         public override bool Equals(object other)
@@ -56,4 +57,4 @@ public override ImmutableArray<PortableExecutableReference> ResolveReference(str
             return result;
         }
     }
-}
\ No newline at end of file
+}
diff --git a/src/OmniSharp.Script/CachingScriptMetadataResolver.cs b/src/OmniSharp.Script/CachingScriptMetadataResolver.cs
index 5ff02eed44..12e092c63f 100644
--- a/src/OmniSharp.Script/CachingScriptMetadataResolver.cs
+++ b/src/OmniSharp.Script/CachingScriptMetadataResolver.cs
@@ -1,4 +1,4 @@
-using System.Collections.Generic;
+using System.Collections.Concurrent;
 using System.Collections.Immutable;
 using Microsoft.CodeAnalysis;
 
@@ -7,8 +7,8 @@ namespace OmniSharp.Script
     public class CachingScriptMetadataResolver : MetadataReferenceResolver
     {
         private readonly MetadataReferenceResolver _defaultReferenceResolver;
-        private static Dictionary<string, ImmutableArray<PortableExecutableReference>> DirectReferenceCache = new Dictionary<string, ImmutableArray<PortableExecutableReference>>();
-        private static Dictionary<string, PortableExecutableReference> MissingReferenceCache = new Dictionary<string, PortableExecutableReference>();
+        private static ConcurrentDictionary<string, ImmutableArray<PortableExecutableReference>> DirectReferenceCache = new ConcurrentDictionary<string, ImmutableArray<PortableExecutableReference>>();
+        private static ConcurrentDictionary<string, PortableExecutableReference> MissingReferenceCache = new ConcurrentDictionary<string, PortableExecutableReference>();
 
         public CachingScriptMetadataResolver(MetadataReferenceResolver defaultReferenceResolver)
         {