diff --git a/src/api/dotnet/UserPropagator.cs b/src/api/dotnet/UserPropagator.cs
index adabe3ff5..0e0ad818a 100644
--- a/src/api/dotnet/UserPropagator.cs
+++ b/src/api/dotnet/UserPropagator.cs
@@ -161,7 +161,7 @@ namespace Microsoft.Z3
         /// </summary>        
         public UserPropagator(Solver s)
 	{
-	    gch = GCHandle.Alloc(this);
+	    gch = GCHandle.Alloc(this, GCHandleType.Pinned);
 	    solver = s;
 	    ctx = solver.Context;
 	    var cb = GCHandle.ToIntPtr(gch);