diff --git a/LanguageExt.Core/Class Instances/Pred/String/Predicates.cs b/LanguageExt.Core/Class Instances/Pred/String/Predicates.cs
index 1965dccf9..9e849b74f 100644
--- a/LanguageExt.Core/Class Instances/Pred/String/Predicates.cs	
+++ b/LanguageExt.Core/Class Instances/Pred/String/Predicates.cs	
@@ -17,7 +17,7 @@ public struct StrLen<NMin, NMax> : Pred<string>
     }
 
     public struct Matches<PATTERN> : Pred<string>
-    where PATTERN : Const<string>
+    where PATTERN : struct, Const<string>
     {
         public static readonly Matches<PATTERN> Is = default;