diff --git a/src/Hyperbee.Expressions/AsyncBlockExpression.cs b/src/Hyperbee.Expressions/AsyncBlockExpression.cs
index 9c57da1..be33b77 100644
--- a/src/Hyperbee.Expressions/AsyncBlockExpression.cs
+++ b/src/Hyperbee.Expressions/AsyncBlockExpression.cs
@@ -51,19 +51,21 @@ public override Expression Reduce()
 
     private LoweringInfo LoweringTransformer()
     {
-        var visitor = new LoweringVisitor();
-
-        var loweringInfo = visitor.Transform(
-            Result.Type,
-            [.. Variables],
-            [.. Expressions],
-            ExternVariables != null ? [.. ExternVariables] : []
-        );
-
-        if ( loweringInfo.AwaitCount == 0 )
-            throw new InvalidOperationException( $"{nameof( AsyncBlockExpression )} must contain at least one {nameof( AwaitExpression )}." );
-
-        return loweringInfo;
+        try
+        {
+            var visitor = new LoweringVisitor();
+
+            return visitor.Transform(
+                Result.Type,
+                [.. Variables],
+                [.. Expressions],
+                ExternVariables != null ? [.. ExternVariables] : []
+            );
+        }
+        catch ( LoweringException ex )
+        {
+            throw new InvalidOperationException( $"Unable to lower {nameof(AsyncBlockExpression)}.", ex );
+        }
     }
 
     protected override Expression VisitChildren( ExpressionVisitor visitor )
diff --git a/src/Hyperbee.Expressions/Transformation/LoweringException.cs b/src/Hyperbee.Expressions/Transformation/LoweringException.cs
new file mode 100644
index 0000000..2342859
--- /dev/null
+++ b/src/Hyperbee.Expressions/Transformation/LoweringException.cs
@@ -0,0 +1,6 @@
+namespace Hyperbee.Expressions.Transformation;
+
+internal class LoweringException : Exception
+{
+    public LoweringException( string message ) : base( message ) { }
+}
diff --git a/src/Hyperbee.Expressions/Transformation/LoweringVisitor.cs b/src/Hyperbee.Expressions/Transformation/LoweringVisitor.cs
index 318bfd3..e9be940 100644
--- a/src/Hyperbee.Expressions/Transformation/LoweringVisitor.cs
+++ b/src/Hyperbee.Expressions/Transformation/LoweringVisitor.cs
@@ -21,11 +21,18 @@ internal class LoweringVisitor : ExpressionVisitor
 
     public LoweringInfo Transform( Type resultType, ParameterExpression[] variables, Expression[] expressions, ParameterExpression[] externVariables )
     {
+        ArgumentNullException.ThrowIfNull( expressions, nameof( expressions ) );
+        ArgumentOutOfRangeException.ThrowIfZero( expressions.Length, nameof( expressions ) );
+
         _variableResolver = new VariableResolver( variables, _states );
         _finalResultVariable = CreateFinalResultVariable( resultType, _variableResolver );
 
         VisitExpressions( expressions );
 
+        StateOptimizer.Optimize( _states );
+
+        ThrowIfInvalid();
+
         return new LoweringInfo
         {
             Scopes = _states.Scopes,
@@ -35,6 +42,17 @@ public LoweringInfo Transform( Type resultType, ParameterExpression[] variables,
             ExternVariables = externVariables
         };
 
+        // helpers
+
+        void ThrowIfInvalid()
+        {
+            if ( _states.Scopes[0].States.Count == 0 )
+                throw new LoweringException( $"Evaluation of the {nameof(expressions)} parameter resulted in empty states." );
+
+            if ( _awaitCount == 0 )
+                throw new LoweringException( $"The {nameof(expressions)} parameter must contain at least one awaitable." );
+        }
+
         static ParameterExpression CreateFinalResultVariable( Type resultType, VariableResolver resolver )
         {
             var finalResultType = resultType == typeof( void )
diff --git a/src/Hyperbee.Expressions/Transformation/StateMachineBuilder.cs b/src/Hyperbee.Expressions/Transformation/StateMachineBuilder.cs
index 6e91915..08a66ae 100644
--- a/src/Hyperbee.Expressions/Transformation/StateMachineBuilder.cs
+++ b/src/Hyperbee.Expressions/Transformation/StateMachineBuilder.cs
@@ -34,16 +34,12 @@ public StateMachineBuilder( ModuleBuilder moduleBuilder, string typeName )
 
     public Expression CreateStateMachine( LoweringTransformer loweringTransformer, int id )
     {
+        ArgumentNullException.ThrowIfNull( loweringTransformer, nameof(loweringTransformer) );
+
         // Lower the async expression
         //
         var loweringInfo = loweringTransformer();
 
-        if ( loweringInfo.AwaitCount == 0 )
-            throw new InvalidOperationException( "The target of a lowering operation must contain at least one await." );
-
-        if ( loweringInfo.Scopes[0].States == null )
-            throw new InvalidOperationException( "States must be set before creating state machine." );
-
         // Create the state-machine builder context
         //
         var context = new StateMachineContext { LoweringInfo = loweringInfo };
@@ -327,10 +323,6 @@ private static IEnumerable<Expression> CreateBody( FieldInfo[] fields, StateMach
 
         var scopes = loweringInfo.Scopes;
 
-        // Optimize source nodes
-
-        StateMachineOptimizer.Optimize( loweringInfo );
-
         // Create the body expressions
 
         var firstScope = scopes[0];
diff --git a/src/Hyperbee.Expressions/Transformation/StateMachineOptimizer.cs b/src/Hyperbee.Expressions/Transformation/StateOptimizer.cs
similarity index 94%
rename from src/Hyperbee.Expressions/Transformation/StateMachineOptimizer.cs
rename to src/Hyperbee.Expressions/Transformation/StateOptimizer.cs
index 902b785..0c11c82 100644
--- a/src/Hyperbee.Expressions/Transformation/StateMachineOptimizer.cs
+++ b/src/Hyperbee.Expressions/Transformation/StateOptimizer.cs
@@ -3,14 +3,14 @@
 
 namespace Hyperbee.Expressions.Transformation;
 
-internal sealed class StateMachineOptimizer
+internal sealed class StateOptimizer
 {
-    internal static void Optimize( LoweringInfo source )
+    internal static void Optimize( StateContext states )
     {
         var references = new HashSet<LabelTarget>();
         int stateOrder = 0;
 
-        var scopes = source.Scopes;
+        var scopes = states.Scopes;
 
         foreach ( var scope in scopes )
         {